Crystal Terminology: An A–Z Guide to Crystal, Mineral

Crystal Terminology: An A–Z Guide to Geological and Mineralogical Terms

A Blog by Luke J. Timberlake

If you’re new to crystals, minerals or geology, you’ve probably come across a few words that sound more like they belong in a university lecture than a hobby. Terms like adamantine lustre, cleavage and crystal twinning can seem intimidating at first, but they don’t have to be.

This glossary aims to explain the most common geological, mineralogical and crystallographic terms in plain English. Whether you’re just starting your collecting journey or you’ve been fascinated by rocks for years, this guide should help you understand the language used by collectors, geologists and gemmologists.

Like geology itself, this glossary will continue to grow over time. I’ll keep adding new entries whenever I come across terms that deserve an explanation, so think of this as a living document rather than a finished one.

If there’s a word or phrase you’d like to see included, leave a comment and let me know. If it’s relevant to crystals, minerals, fossils or geology, I’ll happily add it to the list.

Let’s get started

CRYSTAL TERMINOLOGY

Absolute Dating

The process of determining the actual age of a rock, fossil, archaeological site or geological event, usually expressed in years. Geologists commonly use radiometric dating methods, such as uranium-lead dating, to establish absolute ages.

Adamantine Lustre (See: Lustre)

Adamantine lustre describes the exceptionally bright, brilliant shine seen in gemstones such as diamond. It is one of the highest lustre ratings in mineralogy and is noticeably more dazzling than the vitreous (glass-like) lustre found on many other minerals.

Adder Stone (See: Hag Stone)

An adder stone is a naturally perforated stone with a hole formed by erosion or weathering. Throughout Britain these stones have been associated with folklore and good luck, and are also known as hag stones, witch stones, serpent’s eggs and snake’s eggs. In Wales they are called Glain Neidr, while in parts of Scotland they are known as adderstanes or Gloine nan Druidh (“Druids’ Glass”). In Germany they are referred to as Hühnergötter (“chicken gods”).

Aggregate

A mass or collection of mineral grains or rock fragments that occur naturally together. Many rocks are made up of aggregates of different minerals.

Alkaline

A substance with a pH above 7 that can neutralise acids. In geology, alkaline conditions often influence the formation of certain minerals and rocks.

Amber

Fossilised tree resin that formed millions of years ago. Amber ranges in colour from pale yellow to deep orange and reddish-brown, and often contains beautifully preserved prehistoric insects or plant material.

Amorphous

A material that lacks a regular crystal structure. Unlike true crystals, amorphous substances have atoms arranged randomly. Obsidian is one of the best-known natural examples.

Andalusite

An aluminium silicate mineral commonly formed during the metamorphism of clay-rich rocks. It is prized by collectors for its distinctive crystal shapes and, in some gemstones, its remarkable colour-changing properties.

Asteroid

A rocky object that orbits the Sun, most commonly within the asteroid belt between Mars and Jupiter. Asteroids are important to geology because some meteorites originate from them, while others have collided with Earth, creating impact structures and forming materials such as tektites and Moldavite.

B

Barnacle Crystal

A crystal formation where numerous smaller crystals grow across the surface of a much larger crystal. Some collectors affectionately refer to these as “fairy dust crystals” because of their sparkling appearance.

Biotite (See: Mica)

Biotite is a common black mica mineral found in many igneous and metamorphic rocks, particularly granite. It has perfect cleavage, allowing it to split into extremely thin, flexible sheets.

Bridge Crystal

A naturally formed crystal that connects two separate crystals, creating the appearance of a bridge between them. These unusual formations are highly sought after by many collectors.

C

Carat

A unit of weight used for gemstones, equal to 200 milligrams (0.2 grams). Carat measures a gemstone’s weight, not its size. It should not be confused with karat, which measures the purity of gold.

Calcite

Calcite is the mineral form of calcium carbonate (CaCO₃) and one of the most common minerals on Earth. It occurs in a wide range of crystal habits and forms the main component of limestone, marble and many cave formations.

Cleavage

Cleavage describes the way a mineral naturally splits along planes of weakness within its crystal structure. Even extremely hard minerals such as diamond can be cut relatively easily when struck along their cleavage planes.

Chlorite

A group of green, sheet-like silicate minerals commonly found in metamorphic rocks. Chlorite often forms through the alteration of minerals such as biotite and amphibole during metamorphism.

Continental Crust

The thick, relatively lightweight outer layer of Earth that forms the continents and continental shelves. It consists mainly of granitic, sedimentary and metamorphic rocks.

Compaction

Part of the process of lithification, where sediments become compressed under their own weight, squeezing out water and reducing pore space before eventually forming sedimentary rock.

Composition

In geology and mineralogy, composition refers to the chemical makeup of a mineral or rock—the elements and compounds that combine to form it.

Copal

A young or partially fossilised tree resin that has not fully matured into amber. Copal is often found in tropical regions and can sometimes be mistaken for true amber.

Crystal

A naturally occurring solid whose atoms are arranged in a highly ordered, repeating pattern known as a crystal lattice. This internal structure determines the crystal’s shape, physical properties and crystal system.

Crystal Class

A classification based on the symmetry of a crystal. Mineralogists recognise 32 crystal classes, each describing the symmetry operations that leave a crystal’s structure unchanged.

Crystal System

One of the seven major ways crystals are classified according to the arrangement of their atomic lattice. The seven crystal systems are cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic and triclinic.

Crystal Twinning

A growth feature where two or more crystals share part of the same crystal lattice, producing distinctive intergrown crystal shapes.

Crystallography

The scientific study of crystal structures, including how atoms are arranged within minerals and how those arrangements influence their physical properties.

D

Dacite

A fine-grained volcanic igneous rock rich in silica. Dacite has a composition between andesite and rhyolite and commonly forms during explosive volcanic eruptions.

Double Termination

A crystal that naturally forms with well-developed points at both ends. Because they require uninterrupted growth, naturally double-terminated crystals are generally less common and often more valuable than single-terminated specimens.

E

Enhydro Crystal

An enhydro crystal is a crystal or geode that contains naturally trapped water or other fluids sealed inside as it formed millions of years ago. Enhydros are most commonly found in agate and quartz geodes, but fluid inclusions can also occur within individual crystals such as quartz, topaz, fluorite and gypsum.

Some enhydros contain movable bubbles, making them especially prized by collectors. These bubbles provide a fascinating glimpse into the conditions that existed when the crystal formed. Although genuine enhydro crystals occur naturally, artificial examples are also produced, so careful identification is important.

Faces

The flat surfaces that make up the exterior shape of a crystal. Depending on the crystal habit, a specimen may have just a few faces or many dozens.

Facets

Flat, polished or naturally occurring surfaces on a crystal or gemstone. In crystallography, the arrangement of natural crystal facets reflects the underlying symmetry of the crystal structure, making them an important tool for mineral identification.

Fault

A fracture or crack within a crystal where the crystal lattice has been disrupted. These internal features can refract light, making them appear as visible lines or planes within the specimen. Faults are natural growth features and should not be confused with surface damage.

Feldspar

A large group of rock-forming minerals that make up around half of Earth’s continental crust. Feldspars contain silicon, aluminium and oxygen, together with varying amounts of potassium, sodium or calcium. Well-known varieties include Moonstone, Labradorite and Sunstone.

Fossil

The preserved remains, impression or trace of a prehistoric plant, animal or other organism, typically found within sedimentary rock. Fossils provide valuable evidence of Earth’s ancient environments and the evolution of life.

Fracture

The way a mineral breaks when it does not split along its natural cleavage planes. Different minerals produce distinctive fracture patterns, such as conchoidal (curved, shell-like), uneven or splintery fractures, making this an important feature for mineral identification.

G

Gemstone

A naturally occurring mineral, rock or organic material that has been cut and polished for use in jewellery or ornamentation. Not all crystals are gemstones, and not all gemstones are crystals—for example, amber and pearl are gemstones but are not crystalline minerals.

Gemmology

The scientific study of natural and synthetic gemstones. Gemmology is a branch of mineralogy that focuses on identifying, grading and evaluating gemstones using their physical and optical properties.

Generator

A naturally formed crystal with six sides that taper to a single point. Quartz generators are especially popular with collectors because of their symmetrical shape, although the pointed termination is often delicate and should be handled with care.

Geology

The scientific study of Earth, including its rocks, minerals, fossils, structure, history and the processes that continue to shape our planet. Geology also extends to the study of other rocky planets and moons throughout the Solar System.

Geoscience

Also known as Earth Science, geoscience is the broad study of Earth and its systems. It includes geology, mineralogy, volcanology, palaeontology, oceanography, hydrology and many other disciplines that help us understand our planet.

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Glass

An amorphous (non-crystalline) solid formed when molten material cools so quickly that crystals cannot develop. Natural examples include Obsidian, Moldavite, Libyan Desert Glass and other tektites, while common manufactured glass includes window glass and bottles.

Granite

A coarse-grained intrusive igneous rock composed primarily of quartz, feldspar and mica. Granite forms when magma cools slowly beneath Earth’s surface, allowing large crystals to develop.

H

Habit

A crystal’s characteristic external shape or growth form. Crystal habit is determined by the mineral’s internal atomic structure and the conditions under which it formed. Learning common crystal habits is one of the best ways to identify minerals.

Hag Stone (See: Adder Stone)

A naturally perforated stone with one or more holes formed by erosion. Hag stones feature prominently in British and European folklore, where they were traditionally believed to offer protection or bring good luck.

I

Igneous

Describes rocks that form when molten magma or lava cools and solidifies. Granite, Basalt and Obsidian are all examples of igneous rocks.

Inclusion

A material trapped inside a crystal as it grows. Inclusions may consist of minerals, liquids, gases or tiny fractures and can provide valuable information about the conditions under which the crystal formed. Some inclusions create beautiful optical effects, including rainbows.

Iridescence

The optical phenomenon where a surface appears to change colour depending on the viewing angle or the direction of the light. Labradorite, Ammolite and Peacock Ore are well-known examples of minerals displaying iridescence.

-ite (Suffix)

The suffix “-ite” is commonly added to the names of minerals and many rocks. Derived from the Greek word lithos, meaning “stone”, it has become the standard naming convention for thousands of mineral species, including Calcite, Fluorite and Malachite.

J

Joint

A natural fracture or crack in rock where little or no movement has occurred. Unlike faults, joints do not show significant displacement and commonly form through cooling, pressure release or tectonic stress.

Jurassic

A geological period that lasted from approximately 201 million to 145 million years ago. Famous for its abundance of dinosaurs, the Jurassic also witnessed significant changes in Earth’s climate, oceans and ecosystems.

K

ka

An abbreviation meaning one thousand years. In geology and archaeology, ka usually refers to thousands of years before present (BP). For example, 12 ka means approximately 12,000 years ago.

Kaolinite

A soft white clay mineral composed of hydrated aluminium silicate. Kaolinite forms through the weathering of feldspar-rich rocks and is widely used in ceramics, porcelain and paper production.

Kyanite

A blue aluminium silicate mineral that crystallises in the triclinic crystal system. Kyanite is unusual because its hardness varies depending on the direction in which it is tested, making it distinctive among minerals.

Layered Crystals

Some minerals naturally grow in thin layers or sheets rather than as solid blocks. Lepidolite is a good example, splitting easily into delicate plates because of its perfect basal cleavage.

Lava (See: Magma)

Lava is simply magma that has reached Earth’s surface. As lava cools, it solidifies to form volcanic (extrusive igneous) rocks such as Basalt and Obsidian.

Limestone

A sedimentary rock composed primarily of the mineral calcite (calcium carbonate, CaCO₃). Limestone forms in a variety of ways, including the accumulation of shells, coral and marine organisms over millions of years.

Lustre

Lustre (British English) or luster (American English) describes the way light reflects from the surface of a mineral. Common types include vitreous (glass-like), metallic, pearly, silky, resinous and adamantine, with each helping to identify different minerals.

M

Magma

Molten or partially molten rock found beneath Earth’s surface. As magma cools, it crystallises to form igneous rocks, creating the conditions in which many minerals and crystals develop.

Magnetite

A black iron oxide mineral that crystallises in the cubic crystal system. Magnetite is naturally magnetic, making it one of the easiest minerals to identify.

Mantle

The mantle is the thick layer of hot, mostly solid rock that lies between Earth’s crust and core. Many igneous rocks and minerals ultimately originate from magma generated within the mantle.

Marble

A metamorphic rock formed when limestone is subjected to heat and pressure. Marble consists primarily of recrystallised calcite and is widely used as both a decorative stone and a sculpting material.

Matrix

In geology, the matrix is the material surrounding a crystal, fossil or mineral. For collectors, the matrix is simply the host rock from which a specimen has grown.

Metamorphic Rock

A rock formed when an existing igneous, sedimentary or older metamorphic rock is altered by heat, pressure or chemically active fluids. Slate, schist, gneiss and marble are all examples of metamorphic rocks.

Metal Washed

A treatment in which a crystal or mineral is coated with a thin layer of metal—often titanium or gold—to create an artificial iridescent finish. Although visually striking, metal-washed specimens are modified and should not be confused with their natural appearance.

Metallic Lustre

A type of lustre in which a mineral reflects light in the same way as polished metal. Pyrite, Galena and Hematite are classic examples.

Meteor

A fragment of rock or metal from space that becomes incandescent as it passes through Earth’s atmosphere, producing the familiar “shooting star.” If part of the object survives its journey and reaches the ground, it becomes a meteorite.

Mica (See: Biotite, Muscovite)

A group of sheet silicate minerals known for their perfect cleavage, allowing them to split into incredibly thin, flexible sheets. Common varieties include Muscovite and Biotite.

Mine

An excavation created to extract valuable minerals, metals or rocks from the Earth. Surface mines are often called quarries, while underground mines follow mineral veins beneath the surface.

Mineral

A naturally occurring inorganic solid with a definite chemical composition and an ordered crystal structure. Minerals are the building blocks of rocks and make up much of Earth’s crust.

Mineralogy

The branch of geology dedicated to studying minerals, including their chemistry, crystal structure, physical properties, occurrence and formation.

Mineralogist

A scientist who studies minerals and their properties. If you’ve made it this far through this glossary, you’re already well on your way to thinking like one!

Mohs Hardness Scale

A scale developed by Friedrich Mohs that ranks minerals according to their resistance to scratching, from 1 (Talc) to 10 (Diamond). The Mohs Scale is one of the most useful tools for identifying minerals.

Monoclinic

One of the seven crystal systems. Monoclinic crystals have three unequal axes, with two intersecting at right angles and the third inclined.

Muscovite

The most common light-coloured mica mineral. Muscovite crystallises in the monoclinic system and is easily recognised by its transparent, flexible sheets.

N

Nickel–Strunz Classification

A widely used system for classifying minerals according to their chemical composition and crystal structure. Developed by German mineralogist Karl Hugo Strunz, it is used by mineralogists and museums around the world.

Nodule

A rounded lump of mineral matter found within sedimentary rocks. Nodules commonly consist of silica, calcite, pyrite or gypsum, and may contain fossils or beautiful crystal-lined cavities.

O

Obsidian

A naturally occurring volcanic glass formed when silica-rich lava cools so rapidly that crystals cannot develop. Because it lacks a crystal structure, Obsidian is classified as an amorphous natural glass rather than a true mineral.

Occlusion

A cloudy area or internal imperfection within a gemstone, usually caused by numerous tiny inclusions of liquid, gas or mineral material. Gemmologists often distinguish occlusions from individual inclusions, although the terms are sometimes used interchangeably.

Oolite

A sedimentary rock composed of tiny spherical grains called ooids, which form as layers of calcium carbonate build up around a small particle in warm, shallow marine water.

Opaque

Describes a mineral or crystal that does not allow light to pass through it. An opaque specimen cannot be seen through, regardless of its thickness.

Ore

A naturally occurring rock or mineral that contains valuable metals or minerals in concentrations high enough to be extracted economically.

Organic

Derived from living organisms or biological processes. Amber, pearl and jet are all examples of organic gem materials.

P

Palaeontology

The scientific study of ancient life through fossils. Palaeontologists investigate prehistoric plants, animals and ecosystems to better understand Earth’s history.

Point

A crystal with one naturally formed termination or tip. Crystal points may occur naturally or be cut and polished from larger pieces of rough material.

Pyramid

A four-sided geometric form that may occur naturally in some crystals or be carved from mineral specimens. Although durable in appearance, polished crystal pyramids are prone to chipping at the tip, so careful storage is recommended.

Q

Quartz

One of Earth’s most abundant minerals, composed of silicon dioxide (SiO₂). Quartz occurs in countless varieties, including Amethyst, Citrine, Smoky Quartz and Rose Quartz, and crystallises in the trigonal crystal system.

Quartzose

Describes a rock or material composed largely of quartz or containing a high proportion of quartz.

Quaternary Period

The current geological period, beginning approximately 2.58 million years ago and continuing to the present day. It includes both the Pleistocene (the Ice Ages) and the Holocene, the epoch in which modern human civilisation developed.

R

Rock

A naturally occurring solid composed of one or more minerals or mineraloids. Rocks are broadly classified as igneous, sedimentary or metamorphic, depending on how they formed.

S

Salt Shack

Quite possibly the best place in the world… although we admit we might be slightly biased. Salt Shack is a family-run business built around a passion for crystals, minerals, fossils and geology. One of the owners also decided that writing an A–Z crystal dictionary “wouldn’t take that long”. (See also: Idiot, Oxford English Dictionary.)

Scratch Test

A simple method of identifying minerals by comparing their hardness. The test involves attempting to scratch one mineral with another of known hardness using the Mohs Hardness Scale. Because scratching can damage specimens, it should only be carried out on rough material or where identification is essential.

Sedimentary

Sedimentary rocks form from the accumulation, deposition and cementation of mineral grains, rock fragments or organic material. Common examples include sandstone, limestone and shale.

Silica (See: Quartz)

Silica is the chemical compound silicon dioxide (SiO₂) and is one of the most abundant substances in Earth’s crust. Quartz is the most common naturally occurring crystalline form of silica, although silica can also occur in amorphous forms such as Opal.

Sphere

A perfectly round three-dimensional object in which every point on the surface is the same distance from the centre. Crystal spheres are shaped by polishing rather than natural crystal growth.

Top tip: Dannielle still can’t say the word “sphere” without making everyone laugh.

Streak

The colour of a mineral’s powdered form, produced by rubbing it across an unglazed porcelain streak plate. Streak colour is often more reliable than the mineral’s outward appearance and is an important tool for mineral identification.

Striation

A fine groove, line or scratch found on the surface of a mineral or rock. Striations may form naturally during crystal growth, glacial movement or fault activity, and often provide valuable clues to a specimen’s formation.

Stone

A general term for a piece of naturally occurring rock or mineral. Although the words rock and stone are often used interchangeably, geologists generally use rock when referring to geological materials in situ, while stone usually refers to smaller pieces or material used in construction.

Square Form

A descriptive term often used for minerals that naturally form cubic crystals. Pyrite, Galena and Fluorite commonly display this growth habit. Despite the name, the overall crystal is three-dimensional—so if someone says “square form”, it’s usually best to picture a cube.

T

Tektite

A natural glass formed when a large meteorite impact melts silica-rich rocks and ejects molten material high into Earth’s atmosphere. As the material cools and falls back to Earth, it solidifies into tektites. Famous examples include Moldavite, Indochinites and Libyan Desert Glass.

Tenacity

A mineral’s resistance to breaking, bending, crushing or tearing. Tenacity helps mineralogists distinguish between minerals that may otherwise appear similar.

Topography

The three-dimensional arrangement of the natural and artificial features of an area, including hills, valleys, rivers, cliffs and other landforms.

Tumbled

A stone or crystal that has been polished inside a rotating rock tumbler using progressively finer abrasives. Tumbling smooths rough edges and produces a glossy finish while retaining the stone’s natural colour and pattern.

Tumblestone

A stone or crystal that has been polished in a rock tumbler. Tumblestones are among the most popular forms of crystals because they are smooth, durable and easy to carry.

Tertiary Period

An obsolete geological term formerly used to describe the interval between approximately 66 million and 2.6 million years ago. Modern geology now divides this time into the Palaeogene and Neogene periods.

U

Unit Cell

The smallest repeating three-dimensional building block of a crystal structure. By repeating millions of times in every direction, the unit cell forms the crystal lattice that gives each mineral its unique structure.

UV Reactive

Describes minerals that fluoresce or change appearance under ultraviolet (UV) light. Well-known UV-reactive minerals include Fluorite, Mangano Calcite, Sodalite and some forms of Willemite.

V

Vein

A sheet-like body of mineral material that fills a crack or fracture within surrounding rock. Quartz, Calcite and metallic ore minerals commonly occur in veins.

Vitreous Lustre (See: Lustre)

A type of lustre that resembles the appearance of glass. Quartz, Topaz and many other common minerals display a vitreous lustre.

Volcanic

Describes rocks and minerals formed from lava or volcanic ash at or near Earth’s surface. Because volcanic rocks cool relatively quickly, they often contain small crystals or natural glass.

Volcano

An opening in Earth’s crust through which magma, volcanic ash and gases escape to the surface. Volcanoes are responsible for creating many igneous rocks and mineral deposits found around the world.

W

Wand

An elongated crystal that either formed naturally or has been cut and polished into a wand shape. Crystal wands are popular with collectors because they highlight the natural beauty of elongated minerals such as Quartz.

Weathering

The natural breakdown of rocks and minerals through physical, chemical and biological processes. Weathering is one of the key processes responsible for soil formation and the Earth’s continually changing landscape.

X

X-ray Diffraction (XRD)

A scientific technique used to identify minerals and determine their crystal structure. By measuring how X-rays are diffracted through a crystal, geologists can reveal the arrangement of atoms within the mineral.

Z

Zircon

A zirconium silicate mineral with the chemical formula ZrSiO₄. Zircon is one of Earth’s oldest known minerals, with some crystals dating back over 4.4 billion years. It is highly valued by geologists because it preserves evidence of the Earth’s earliest history and plays a vital role in radiometric dating.

Conclusion

We hope this glossary has helped make the sometimes bewildering world of crystals, minerals and geology a little easier to understand. Whether you’re just starting your collecting journey or you’ve been fascinated by rocks for years, there’s always something new to learn.

This glossary will continue to grow as we add new terms and update existing definitions. If you think we’ve missed something, leave us a comment—we’re always happy to expand the list.

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