gem.
For it presents a considerable range of rich as well as of delicate
hues; its surface lustre is brilliant, almost adamantine ; while its
chemical composition is rendered noteworthy through the predominant
presence of the rare earth zirconia. Although zircon, even in its most
richly coloured varieties, is but feebly dichroic, yet some specimens
display a considerable amount of " fire," owing to their high
dispersive power, while the majority depend for their brilliancy mainly
upon their strong refraction of light, which approaches that of
diamond. Moreover the spectroscope reveals the presence, in many of the
transparent specimens, of a series of black absorption bands
(discovered by the author in 1866), which are characteristic, and have
been attributed to small quantities of a uranium compound, to erbium
and to an unknown element.* Zircon crystallises in the tetragonal
system, and usually occurs in short square prisms terminated by
pyramids containing facets of tetragonal octohedra. Its hardness is
about 7.5, but rather lower than this figure in the varieties of lower
density. The specific gravity of zircon demands special consideration,
as this physical property presents, in the case of this mineral,
features of quite singular interest. In fact the specific gravity of
different specimens ranges from 3.98 to 4.86, a range more extensive
than that of any other precious stone, indeed of any other mineral. Of
the gem-varieties of zircon the least dense are those which are of a
pure leaf-green colour, 3.98 to 4.8 ; then come the paler and duller
green stones, 4.2 to 4.4 ; then the clouded orange or gold-coloured
stones, 4.32 to 4.45 ; then the great bulk of yellow, orange, red,
brown, puce, and white stones, 4.6 to 4.75 ; the flame-red zircons of
Expailly, Auvergne, with a density approaching 4.86 conclude the
series. The following set of determinations of the specific gravity of
flawless cut specimens of zircon (both jargoons and hyacinths) will be
useful for reference, particularly as the numbers represent the values
obtained
