lustre, the hard powder of which is used in polishing diamonds. It is almost pure anhydrous alumina (Al2O3)
and is, roughly, four times as heavy as water. The lustre of this is
the true " adamantine," or diamond, brilliancy, and the other and
impure divisions of this particular lustre are : splendent, when
objects are reflected perfectly, but of a lower scale of perfection
than the true "adamantine " standard, which is absolutely flawless.
When still lower, and the reflection, though maybe fairly good, is
somewhat " fuzzy," or is confused or out of focus, it is then merely shining ;
when still less distinct, and no trace of actual reflection is possible
(by which is meant that no object can be reproduced in any way to
define it, as it could be defined in the reflection from still water or
the surface of a mirror, even though imperfectly) the stone is then
said to glint or glisten. When too low in the scale even
to glisten, merely showing a feeble lustre now and again as the light
is reflected from its surface in points which vary with the angle of
light, the stone is then said to be glimmering. Below this, the definitions of lustre do not go, as such stones are said to be lustreless.
(b) Pearly, as its name implies, is the lustre of a pearl.
(c) Silky, possessing the sheen of silk, hence its name.
(d) Resinous, also explanatory in its name ; amber and the like come in this variety.
(e)
Vitreous. This also explains itself, being of the lustre of glass,
quartz, etc. ; some experts subdividing this for greater defining
accuracy into the " sub-vitreous " or lower type, for all but perfect
specimens.
