thus:--a
"non-diaphanous" stone is one which is quite opaque, no light of any
kind passing through its substance; a "diaphanous" stone is one which is
altogether transparent; "semi-diaphanous" means one not altogether
transparent, and sometimes called "sub-transparent." A "translucent"
stone is one in which, though light passes through its substance, sight
is not possible through it; whilst in a "sub-translucent" stone, light
passes through it, but only in a small degree.

The second physical property of light is seen in those stones which owe
their beauty or value to REFLECTION: this again may be dependent on
Lustre, or Colour.

~Lustre.~--This is an important characteristic due to reflection, and of
which there are six varieties:--([alpha]) adamantine (which some
authorities, experts and merchants subdivide as detailed below);
([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon])
vitreous; ([zeta]) metallic. These may be described:--

([alpha]) Adamantine, or the peculiar lustre of the diamond, so called
from the lustre of adamantine spar, which is a form of corundum (as is
emery) with a diamond-like lustre, the hard powder of which is used in
polishing diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3})
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_.

([beta]) Pearly, as its name implies, is the lustre of a pearl.

([gamma]) Silky, possessing the sheen of si