U

Ultrabasic

Describing an igneous rock containing magnesium and iron with a very low percentage of silica (less than 45%)

Ultraviolet

Pertaining to ultraviolet light.

Ultraviolet light

Type of electromagnetic radiation which cannot be seen by humans.

Uneven (fracture)

Fracture that leaves a rough or irregular surface.

Unstable

Mineral with a crystal structure that is unable to exist at the current temperature. When certain minerals become unstable, they transform into their paramorphs. Also refers to radioactive minerals which cannot withstand their composition and begin to decay.

U.V.

Abbreviation of ultraviolet