Aggregate →
A mass of mineral crystals or grains that are grouped together, rather than a single crystal.
Example: Botryoidal (grape-like) hematite is an aggregate of many tiny crystals.
Glossary
Explore the fascinating properties, uses, and locations of Earth's most remarkable minerals.
A
Asterism →
A star-like luminous effect seen in some crystals when light is reflected from inclusions of needle-like crystals (usually rutile).
Example: Star Sapphires and Star Rubies are prized for their perfect six-rayed asterism.
C
Cleavage →
The tendency of a mineral to break along flat, smooth surfaces determined by its internal crystal structure. Cleavage planes are areas of weak atomic bonding.
Example: Mica has perfect cleavage in one direction, allowing it to split into thin, flexible sheets.
Crystal System →
The classification of crystals based on their symmetry and geometric properties. There are seven crystal systems: cubic, hexagonal, tetragonal, orthorhombic, monoclinic, triclinic, and trigonal.
Example: Halite (table salt) crystallizes in the cubic system, forming perfect cube-shaped crystals.
E
Effervescence →
The bubbling or 'fizzing' reaction that occurs when a carbonate mineral reacts with a weak acid (like hydrochloric acid or vinegar), releasing carbon dioxide gas.
Example: Calcite can be easily identified by its strong effervescence when a drop of vinegar is applied.
F
Fluorescence →
The emission of visible light by a mineral when exposed to ultraviolet (UV) light. The light stops when the UV source is removed.
Example: Many fluorite specimens glow blue or green under UV light.
Fracture →
The pattern in which a mineral breaks when it doesn't break along cleavage planes. Common types include conchoidal (shell-like), uneven, and splintery.
Example: Obsidian shows conchoidal fracture, breaking with smooth, curved surfaces like broken glass.
G
Gemstone →
A mineral or organic material that has been cut and polished for use in jewelry or decoration. Valued for beauty, rarity, and durability.
Example: Diamond, ruby, sapphire, and emerald are precious gemstones.
Geode →
A hollow, rounded rock containing crystals that grew inward from the walls. Often lined with quartz or amethyst crystals.
Example: Amethyst geodes from Brazil can contain spectacular purple crystal formations.
H
Habit →
The characteristic external shape of an individual crystal or a group of crystals, such as prismatic, acicular (needle-like), or botryoidal (grape-like).
Example: Quartz often shows a prismatic habit, forming six-sided columns.
Hardness →
A measure of a mineral's resistance to scratching, typically measured using the Mohs scale (1-10). Diamond is the hardest natural mineral at 10.
Example: Quartz has a hardness of 7, meaning it can scratch glass but can be scratched by topaz.
Hydrothermal →
Relating to the action of hot water, especially in the Earth's crust. Many important mineral deposits form from hydrothermal solutions.
Example: Quartz and gold often precipitate from hydrothermal fluids circulating through rock fractures.
I
Igneous →
Rocks or minerals formed from the cooling and solidification of molten magma or lava.
Example: Quartz often forms in igneous rocks like granite.
Inclusion →
Any material that is trapped inside a mineral during its formation. Inclusions can be other minerals, gas bubbles, or liquids.
Example: Rutilated quartz contains needle-like inclusions of the mineral rutile.
L
Luminescence →
The emission of light by a mineral that is not caused by high temperatures. It includes fluorescence (under UV light) and phosphorescence (continuing after UV is removed).
Example: Fluorite and some Calcite specimens exhibit beautiful luminescence when exposed to ultraviolet light.
Luster →
The way light reflects from the surface of a mineral. Common types include metallic, vitreous (glassy), pearly, silky, and dull.
Example: Pyrite has a metallic luster, while quartz has a vitreous (glassy) luster.
M
Metamorphic →
Rocks or minerals formed by the transformation of existing rocks under high temperature and pressure conditions.
Example: Garnet commonly forms in metamorphic rocks like schist and gneiss.
Mineraloid →
A mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals.
Example: Opal and obsidian are common mineraloids because they lack an ordered crystal structure.
Mohs Scale →
A scale of mineral hardness from 1 (talc) to 10 (diamond), created by Friedrich Mohs. Each mineral can scratch those below it on the scale.
Example: The Mohs scale: 1-Talc, 2-Gypsum, 3-Calcite, 4-Fluorite, 5-Apatite, 6-Orthoclase, 7-Quartz, 8-Topaz, 9-Corundum, 10-Diamond.
N
Native Element →
A mineral that is composed of atoms of a single element, occurring naturally in an uncombined form.
Example: Gold, Silver, Copper, and Diamond (Carbon) are all native elements.
O
Ore →
A naturally occurring mineral or rock from which a metal or valuable mineral can be profitably extracted.
Example: Galena is an ore of lead, while chalcopyrite is an important ore of copper.
P
Piezoelectric →
The property of certain minerals to generate an electric charge when subjected to mechanical stress, or to deform when an electric field is applied.
Example: Quartz is piezoelectric, which is why it's used in watches and electronic oscillators.
Polymorph →
Two or more minerals that have the same chemical composition but different crystal structures and physical properties.
Example: Diamond and graphite are polymorphs: both are pure carbon, but their atoms are arranged differently.
S
Sedimentary →
Rocks formed from the accumulation and cementation of mineral and organic particles, often in water environments.
Example: Halite (rock salt) forms when seawater evaporates, leaving behind sedimentary deposits.
Silicate →
The largest group of minerals, characterized by silicon-oxygen tetrahedra (SiO₄) as their basic structural unit. Silicates make up about 90% of Earth's crust.
Example: Quartz, feldspar, and mica are all silicate minerals.
Specific Gravity →
The ratio of a mineral's weight compared to an equal volume of water. It indicates how dense a mineral is.
Example: Gold has a specific gravity of about 19.3, making it much heavier than most minerals.
Streak →
The color of a mineral in powdered form, typically observed by rubbing the mineral on an unglazed porcelain plate.
Example: Hematite appears black or silver but has a distinctive red-brown streak.
T
Tenacity →
The resistance of a mineral to breaking, bending, or deforming. Terms like brittle, sectile, malleable, and elastic describe a mineral's tenacity.
Example: Copper is malleable, meaning it can be hammered into thin sheets, while quartz is brittle and shatters easily.
Transparency →
The degree to which light passes through a mineral. Minerals can be transparent (clear), translucent (some light passes through), or opaque (no light passes through).
Example: Clear quartz is transparent, while rose quartz is translucent, and hematite is opaque.
V
Vein →
A sheet-like deposit of minerals that fills a fracture or crack in a host rock, often formed by hydrothermal fluids.
Example: Gold is often found in quartz veins that cut through surrounding rock.