Glossary
The terms you will meet in shops, on reports and in our guides, defined clearly and without sales language.
- Adularescence
- The floating blue-white sheen of moonstone.
- Assay
- The testing of a metal to determine its composition and purity.
- Asterism
- The star effect seen in some sapphires and rubies.
- Brilliance
- The light returned to the eye from within a gemstone.
- Cabochon
- A gemstone cut with a smooth domed top rather than facets.
- Carat
- The standard unit of gemstone weight, equal to 0.2 grams.
- Ceylon sapphire
- A sapphire of Sri Lankan origin.
- Chatoyancy
- The cat’s-eye effect: a single bright band of light across a cabochon.
- Clarity
- The degree to which a gemstone is free of visible inclusions.
- Colour zoning
- Uneven distribution of colour within a gemstone.
- Corundum
- The mineral species that includes sapphire and ruby.
- Cut
- The proportions, symmetry and finish of a fashioned gemstone.
- Diffusion treatment
- A treatment that introduces colouring elements into a gem’s surface at high temperature.
- Extinction
- Dark, lifeless areas in a faceted gem face-up.
- Eye-clean
- Showing no inclusions visible to the unaided eye.
- Fineness
- Gold or silver purity expressed in parts per thousand, such as 916 or 750.
- Fracture filling
- Filling surface-reaching fissures with glass or resin to improve apparent clarity.
- Gem gravel (illam)
- The gem-bearing gravel layer worked in Sri Lankan alluvial mining.
- Gem report
- A laboratory document identifying a gemstone and disclosing treatments.
- Geuda
- Milky, desaturated corundum that can turn fine blue when heated.
- Hallmark
- An official mark stamped on precious metal indicating its purity.
- Heated sapphire
- A sapphire whose colour or clarity has been improved by controlled heating.
- Illam
- The Sri Lankan term for gem-bearing gravel.
- Imitation
- Any material that merely looks like the gem it represents.
- Inclusion
- A natural internal feature within a gemstone.
- Karat
- A measure of gold purity on a 24-part scale; 24K is pure gold.
- Lapidary
- A craftsperson (or workshop) that cuts and polishes gemstones.
- Making charge
- The labour and workmanship fee added to the metal value of jewellery.
- Mohs scale
- A 1–10 scale of relative scratch hardness for minerals.
- Origin report
- A laboratory opinion on the geographic origin of a gemstone.
- Padparadscha
- A rare pink-orange sapphire historically associated with Sri Lanka.
- Pleochroism
- A gem showing different colours when viewed from different directions.
- Point (gem weight)
- One hundredth of a carat, or 2 milligrams.
- Refractive index
- A measure of how strongly a gem bends light; a key identification property.
- Rose gold
- A gold-copper alloy with a pink colour.
- Ruby
- The red gem variety of corundum.
- Sapphire
- Gem-quality corundum in any colour except red.
- Silk
- Fine needle-like rutile inclusions in corundum.
- Sovereign (gold weight)
- A traditional gold weight unit of 8 grams used in South Asian jewellery trade.
- Specific gravity
- A gem’s density relative to water; used in identification.
- Spot price
- The current market price for immediate delivery of a metal.
- Sterling silver
- A silver alloy of 92.5% silver, marked 925.
- Synthetic gemstone
- A man-made crystal with the same composition as its natural counterpart.
- Treatment
- Any human process, beyond cutting and polishing, that alters a gemstone’s appearance.
- Wastage
- A traditional charge for metal lost during manufacturing.
- White gold
- A gold alloy whitened with metals such as palladium or nickel.
- Window
- A washed-out see-through area in the centre of a poorly proportioned stone.
- Windowing
- A see-through area in the middle of a poorly proportioned gem.