D

Decomposition

The molecular breakdown of certain minerals which cause a mineral to disintegrate. (i.e. Orpiment)

Decrepitation

An explosive shattering of minerals, usually through tube tests or blowpipe tests.

Dehydration

The removal of water from a substance. Many minerals naturally lose water in their structure at normal conditions, and the mineral transforms into another mineral (i.e. Torbernite transforms to Meta-Torbernite by loss of water in its structure).

Dendrite

Mineral with a dendritic shape.

Dendritic

Aggregate composed of skeletal or tree-like formations. May be a single entity, or a formation that forms from mineral-rich solutions that deposit the mineral in rock and form a tree or plant structure embedded in rock. There sometimes is a distinction noted between the two aggregates; in some guides the former aggregate (single entity) is known as skeletal, and the latter as dendritic (embedded in rock). Other guides, such as this one, don't distinguish the two, and term them both as dendritic.

Deposit

An accumulation of certain minerals within a rock formation.

Detritus

Minerals or rock fragments that eroded and end up in a different region from natural causes, such as downstream currents.

Devitrification

Changing over from a natural glass to a mineral with a crystalline structure.

Devitrify

The process of a natural glass to lose its glassy nature and crystallize.

Diagenesis

The final stage of changing over from sediment into sedimentary rock.

Diamagnetism

Magnetic property which causes a mineral to be repelled from magnetic fields.

Diaphaneity

The quality of of a substance to be seen through. In regard to minerals, it is variable with transparency.

Diaphanous

Able to be seen through, being either transparent or translucent

Dichroic

Displaying dichroism. Minerals that exhibit dichroism are said to be dichroic.

Dichroism

Literally means "two colors". A mineral that exhibits one color when viewed from one angle but a different color when viewed from a different angle is said to display dichroism. (long green tourm)

Diffraction

The bending of light when it enters from one medium into another. For example, light bends as it travels from air into another substance, such as water. Diffraction also occurs when light enters from the air into a mineral, and the amount of diffraction varies among minerals.

Dike

A wall of igneous rock cutting across layers of surrounding rock.

Dipyramid

In form with a plane dividing a crystal into two pyramids base to base. Synonym of bipyramid. (sketch). (i.e. Zircon)

Dispersion

The splitting of white light into the colors of the spectrum. When white light enters a crystal, it splits up; some rays are reflected and some are absorbed, forming the mineral's color. Gemstones are cut in a fashion that the split-up light in the crystal fans out its colors and reflects from an exact face. This causes "fire" in a gem, which is best seen on colorless gems.

Dodecahedron

Twelve sided polyhedron; all sides are equidimensional and either rhombic or pentagonal. If the dodecahedron is composed of rhombs, it is known as a rhombic dodecahedron, or simply as a dodecahedron. If it is composed of pentagons, it is known as a pentagonal dodecahedron or pyritohedron. Minerals shaped as dodecahedrons belong to the isometric system. Plural = Dodecahdra or Dodecahedrons

Dollar

Flat, spherical disc of radiating crystals. Also a variety of Pyrite / Marcasite.

Double refraction

Phenomenon exhibited on all non-opaque minerals except for amorphous ones and ones that crystallize in the isometric system. A light ray enters the crystal and splits up into two separate rays, making anything observed through the crystal appear as double. The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, in some minerals, such as the Iceland Spar variety of Calcite it is strongly seen. The double refraction is different in every mineral, and thus can be used to identify gems. Double refraction is measured with a refractometer

Doublet

One of the many gem fakes in which a thin, flat section of a real gem is pasted atop a thick base of glass or rock crystal.

Druse

Cavity in a mineral or rock filled with protruding crystals. The hole is either completely filled with crystals or just partially. (i.e. Adamite, Trap rock zeolites)

Drusy

Aggregate composed of prismatic crystals protruding from a cavity or wall.

Dry lake

Saline lake that evaporated or was drained. Dry lakes leave over many evaporite minerals, including salts, borates, and nitrates. See Dry lake deposit.

Dry lake deposit

Deposit containing an accumulation of evaporite minerals from the evaporation or drainage of a saline lake. As the water gets exhausted, the minerals it is rich in remain, increasing in content, and eventually all that is left is accumulation of the mineral that was once present in the water. See also dry lake and saline deposit.

Ductile

Capable of being stretched into a thin wire. A form of tenacity.

Ductility

The capability of being able to stretched into a thin wire; being ductile.

Dull (luster)

The luster of minerals with very poor optical properties.

Dump

Area where left over material is placed after being extracted from a mine.

Dusting

Very thin coating of one mineral on another mineral; a very thin sprinkling.