These are all visual characteristics assessed by simple observation of the stone with light.
Colour is a very important criterion for identifying gemstones, as it will be a determining factor for certain variety names: blue corundum, for instance, is called sapphire. Colour also helps in the differentiation of mineral species, since some gemstones do not occur in certain colours: garnet, for example, never occurs in blue. Colour is finely evaluated by stating the main visible hue (blue, green, yellow, orange, red, etc.) as well as a colour modifier (bluish, yellowish, reddish, purplish, etc.). The intensity of the colour (saturation) and its lightness or darkness (tone) are additional features describing the colour of a gemstone.
Transparency describes the amount of light able to pass through the gemstone. Most gemstones are transparent while a few are semi-transparent, translucent or opaque.
Lustre is the appearance of a material surface in reflected light. It is best assessed where the polish is as fine as possible. A metallic lustre, the highest lustre of all, displays mirror-like reflection. Lustre is another clue in the differentiation of gem species. The following stones are in decreasing order of lustre: hematite (metallic), diamond (adamantine), tourmaline (vitreous), amber (resinous).
Dispersion is the ability to split white light into its basic spectral colours (the colours of the rainbow). In gemstones, this is translated into flashes of colour when the stone or the light source moves. Not all gemstones exhibit this property.
Diamond is famous for its fire, which is the term given to the appearance of its moderate, yet highly noticeable, dispersion. Garnets such as andradite, when light enough, also show moderate dispersion. Ruby and sapphire, on the other hand, exhibit a weaker dispersion which is not normally visible.
Phenomenon is the term used to designate a peculiar optical effect observed in gemstones. Phenomena can have various origins, and are relatively uncommon. These include: chatoyancy, labradorescence, play-of-colour, asterism, adularescence, change-of-colour, aventurescence, iridescence and orient.
Phenomena usually add value to gemstones and can also guide the gemmologist in the identification process. Indeed, some optical effects are present in some mineral species, such as star sapphire, star diopside, star beryl, star garnet, star quartz, while some others are virtually unique to a mineral species, such as the play-of-colour of precious opal.
