Glossary
| Adamantine | A brilliant lustre, like polished diamond |
| Anion | Negatively charged ion |
| Asteria | Form of ruby or sapphire containing a star shape (usually six-pointed). Caused by the reflection of light from needle-like rutile inclusions. Also known as a star-stones |
| Birefringence | The ability of a mineral to split plane polarised light into two individual rays |
| Cabochon | The dome-shaped cut of sapphire or ruby required to see the star shape in asterias |
| Cation | Positively charged ion |
| Cleavage | The ability of a mineral to split apart cleanly along certain crystal planes |
| Corundum | Mineral: crystal form of alumina (Al2O3) |
| Crystal Habit | The external form of a crystal |
| Crystal System | One of the seven groups crystals can be placed in, describing the dimensions of the unit cell |
| Dichroism | The property that some birefringent minerals have of absorbing different wavelengths of light in the two different beams. This gives rise to two separate colours |
| Feed Powder | The powdered mixture of ingredients used in the Verneuil Technique |
| Flux-growth | A method developed after the Verneuil Technique for synthesising rubies and sapphires, which produces good optical properties in the gemstones |
| Geuda | A form of corundum, varying from milky white to brown in colour, containing rutile inclusions. Used in the artificial synthesis of sapphires in a method developed in the 1970s |
| Hexagonal Close Packing (hcp) | The packing of spheres so as to occupy the least amount of space. In a plane, each sphere is surrounded by six others, in a hexagonal arrangement. The spheres in the second plane fit into the depressions in the first. In hcp, the arrangement of planes is ABABAB, meaning that the spheres in the third plane are directly beneath those in the first. |
| Lustre | The way in which a mineral reflects light |
| Ruby | Red form of the mineral corundum. Colouration caused by chromium impurities |
| Sapphire | Non-red coloured form of the mineral corundum. Colouration caused by trace metal impurities (blue: Ti4+ and Fe2+) |
| Streak | The colour made by the mineral when scraped across an abrasive surface |
| Verneuil Technique | The industrial process by which rubies and sapphires are artificially synthesised |
| Vitreous | A glassy lustre |
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