See specific gravity
When the star is best visible in transmitted light, example: star rose quartz
See also asterism
Opposite of epiasterism
Bi-colouredness, a form of pleochroism
Optical instrument for the separation of the different colours of pleochroic gems. Makes use of the fact that doubly refractive stones split light into two rays polarized at right angles to each other
The fire of gemstones caused by the splitting of light into its spectral colours upon reflection at two surfaces inclined to each other. Very prominent in demantoid garnet, diamond, white zirkon e.a.
The table facet of seal stones is sometimes very slightly domed
Doubly refractive minerals split lightrays in two upon entry, opposite: single refraction
A composite stone consisting of two parts; common in opals to make use of opal slices of high quality that would otherwise be too thin. Sometimes used to defraud e.g. garnet/blue glass doublets or synth. blue sapphire/colourless sapphire doublets may imitate natural blue sapphire. Difficult to detect in closed settings (gas bubbles in cement layer)