Ultrabasic
It describes an igneous rock which contains less than 45% of silica (SiO₂), including most ultramafic rocks.

Ultramafic
It is composed chiefly of ferromagnesian (Fe-Mg) minerals, like olivine and pyroxene.

Ultraviolet
Pertaining to ultraviolet light.

Ultraviolet lamp
See fluorescent lamp.

Ultraviolet light
It is the light that is invisible to the naked eye as it consists of shorter wavelengths than those of visible light.

Undercut
It is a cut which presents an overhang part, as a drip mold.

Uneven (fracture)
It is the fracture which leaves a rough or irregular surface.

Unstable
It is term given to a mineral having crystal structure being unable to exist at the current temperature. When minerals become unstable, they transform to their paramorphs. It is also refered to radioactive minerals that do not withstand their composition and begin to decay.

U.V.
Abbreviation of ultraviolet.

U.V. lamp
It is the abbreviation of ultraviolet lamp.