Gemstone optical terms explained in plain English

What should you look for when a description promises lustre, fire or a glow? Translate each word into an observation before trying to judge the stone. This small glossary is meant to make a viewing conversation easier. It does not provide a way to identify an unknown gem at home.

Surface reflection and light passing through

Lustre describes how a surface looks as it reflects light. “Vitreous” means glass-like; “adamantine” means diamond-like. These are descriptive terms. A polished surface and a broken surface may show different lustre, so context matters. IGS on gemstone lustre

Transparency concerns the light transmitted through a gem. Transparent material lets you see an image through it. Translucent material passes light but scatters it enough to obscure the image. Opaque material does not transmit visible light. These appearances alone are insufficient for identifying a species. IGS on transparency

When a description uses either term, ask the seller to show the feature it refers to. You might request a surface view for lustre and a second view with light behind the stone for transmission. Describe what you see before adopting a more technical label.

Diagram text

Reflection: Lustre concerns the surface. Transmission: Transparency concerns light passing through. Opaque example: No transmitted visible light.

Brightness fire and scintillation

In diamond descriptions, fire refers to coloured flashes produced by dispersion, the separation of white light into spectral colours. Scintillation includes flashes and the pattern of light and dark as the diamond moves. Lighting changes their appearance. A single bright photograph cannot show every part of this performance. GIA on light and diamond appearance

“Brilliance” is used less consistently in coloured-stone discussions. GIA’s cut specialist explains that some speakers mean light return, while others mean the lively appearance or returned colour. Ask which meaning the seller intends before comparing two descriptions. GIA on coloured-stone brightness

Directional colour and glow

Iridescence describes a display of colours produced by structures on or within a material. Opal’s play-of-colour is associated with its microscopic ordered spheres. These terms concern optical effects rather than the material’s main colour. Gem-A on iridescence

Adularescence is the silvery or bluish sheen associated with moonstone. It is directional: illumination and viewing angle determine whether you see it. Ask to see the glow from the viewing position intended for the finished jewellery. Gem-A on moonstone

For stars and bright bands, read the star and cat’s-eye viewing guide. For a whole-stone colour comparison under different lamps, use the colour-change guide. Keep the term, the viewing setup and your observation together in your notes. That gives the vocabulary a job instead of letting it stand in for a quality judgment.