|
28
|
PRECIOUS STONES
|
||
|
|
|||
|
rotated, it fails at every point to
transmit light, which a double-refracting gem allows to pass except
when its optical axis is placed in the axis of the polariscope, but
this will be dealt with more fully when the methods of testing the
stones come to be considered.
Diaphaneity, or
the power of transmitting light:— some rather fine trade distinctions
are drawn between the stones in this class, technical distinctions made
specially for purposes of classification, 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 RE flection : 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 :—(a) adamantine (which some authorities, experts and
merchants subdivide as detailed below) ; (b) pearly; (c) silky ; (d)
resinous; (e) vitreous ; (f) metallic. These may be described :—
(a)
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
|
|||
|
|
|||