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density
see specific gravity

diasterism
when the star is best visible in transmitted light, example: star rose quartz
also see asterism

opposite of epiasterism

dichroism
bi-colouredness, a form of pleochroism

dichroscope
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

dispersion
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.

domed table
the table facet of seal stones is sometimes very slightly domed

double refraction
doubly refractive minerals split lightrays in two upon entry, opposite: single refraction

doublet
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)

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