substance, but by its peculiar property of refracting the solar rays. It
is a compound of about 90 silica, and 10 water. The finest specimens
come exclusively from Hungary. There is a variety of opal called
_Hydrophane_, which is white and opaque till immersed in water; it then
resembles the former.

_Why is the sapphire genus so highly prized?_

Because, after diamond, it is the hardest substance in nature. It forms
also the most valuable gems, as the oriental ruby and the topaz. The
blue variety, or sapphire, is harder than the ruby. It is infusible
before the blowpipe. It becomes electrical by rubbing, and retains its
electricity for several hours; but does not become electrical by
heating. It occurs in alluvial soil, in the vicinity of rocks belonging
to the secondary or floetz-trap formation, and imbedded in gneiss. It is
found at Rodsedlitz and Treblitz in Bohemia, and Hohenstein in Saxony;
Expailly in France; and particularly beautiful in the Capelau mountains,
twelve days from Sirian, a city of Pegu. Next to diamond it is the most
valuable of gems. The white and pale blue varieties, by exposure to heat
become snow-white; and when cut, exhibit so high a degree of lustre,
that they are used in place of diamond. The most highly prized varieties
are the crimson and carmine red; these are the oriental ruby of the
jeweller; the next is sapphire; and the last is sapphire, or oriental
topaz. The asterias, or star-stone, is a very beautiful variety, in
which the colour is generally of a reddish violet, with an opalescent
lustre. A sapphire of ten carats weight is considered to be worth fifty
guineas.--_Jameson_.

The blue topaz, or Brazilian sapphire, is of recent introduction. The
white topaz considerably exceeds rock crystal in lustre, and in Brazil