A distinctive pattern of dark absorption lines and broad absorption bands observed when light reflected from, or transmitted through, a gemstone is viewed through a spectroscope.

Igneous rocks with a high silica content and usually light in colour.

The highest classification of surface lustre or reflectivity which may be shown by a gemstone e.g. in faceted diamond.

A white/blue shimmer which passes across the curved surface of a polished moonstone cabochon due to scattering of light from alternating parallel layers of potassium and sodium rich feldspars.

Colouring due to a trace impurity which is not part of the basic chemical composition of the gem mineral or colouring resulting from coloured inclusions.

Mineral deposits transported by streams from their original source and deposited at sites of lower fluid velocity.

Without crystalline structure or definite shape e.g. glass, amber, precious opal.

A term used to describe crystals which do not have well formed faces.

Irregular light and dark patches observed when singly refracting stones having internal strain are rotated under the polariscope e.g. diamond, garnet and some pastes. Synthetic spinel shows stripes ca [..]

"When a body is immersed in a fluid, there is an apparent loss of weight equal to the weight of the fluid displaced".