OUTWARD CHARACTERISTICS. TRANSPARENCY.
Transparency is the property that precious stones possess of being more or less easily traversed by luminous rays.
They are transparent when,
being interposed between the eye and an object, they allow all the
outlines of this object to be seen with perfect clearness. Example:
the diamond.
They are semi-transparent when the objects viewed through them are a little confused. Example: the emerald.
They are translucid when
nothing can be perceived when they are placed before the eye, but that
the light evidently has a passage through them. Example: the chalcedony.
Finally, they are opaque when not a ray of light can penetrate them. Example: the jasper.
LUSTRE.
"
Among minerals great differences are met with as regards the manner in
which the light acts upon their surface. In this respect there are two
separate effects to be distinguished, lustre and colour, which are one to the other as timbre is to sound in a
