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xiv A Necessary Introduction
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occur in crystalline and those which occur in noncrystalline forms. Diamond is crystalline, agate is not.
Again,
the crystalline forms may be cubic, tetragonal, hexagonal, rhombic,
monocUnic or triclinic, according to natural crystalhsation, and this
crystalline form is as important to the beauty of gems as their colour,
because it determines the way in which they reflect light.
A
third point is that the degree of hardness is important in determining
a stone's place in the peerage of gems. The scale of hardnesses drawn
up by the mineralogist Mohs is one which is accepted as the standard by
which gems as well as other minerals are judged for this quality. In
this scale, certain representative stones are ranked as follows:
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Lustre,
a quality of dispersing light, is of paramount importance in those gem
stones which by nature are not opaque or semi-opaque. Lustrous stones
may be sub-translucent, translucent, sub-transparent or transparent.
Varying lustres are described by various allusive adjectives, such as
"adamantine" (with the quality of diamond), "resinous", "vitreous",
"pearly" or "waxy".
Many people do not realise that practically every colour, tint and shade may be met with in every species
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