(_From Part 15, of Knowledge for the People--Mineralogy and Geology_.)
_Why was crystal so named?_
Because it was probably the first substance ever noticed as occurring in
a regular form, and the ancients believing it to be water permanently
congealed by extreme cold, from its transparency, called it
_Krustallos_, signifying ice; but in time the word became used without
attention being paid to its original meaning, and was applied to all the
regular figures observed in minerals.
_Why are the fine crystals of quartz used as a substitute for glass in
spectacles?_
Because, from their superior hardness, they do not so readily become
scratched as glass: they are then termed pebbles.
_Why is the stone Cairn Gorm so called?_
Because it is found in great beauty in the mountain of Cairn Gorm, in
Scotland. It consists of brown and yellow crystals of quartz, and is
much admired for seal stones, &c.; it is sometimes improperly termed
topaz.
_Why is quartz the constituent of so many gems?_
Because the tinges it receives from metals are sufficient to produce
these varieties. Thus, _amethyst_, or purple quartz, is tinged with a
little iron and manganese. _Rose quartz_, or false ruby, derives its
colour from manganese. _Avanturine_ is a beautiful variety of quartz, of
a rich brown colour, which, from a peculiarity of texture, appears
filled with bright spangles. Small crystals of quartz, tinged with iron,
are found in Spain, and have been termed _hyacinths of Compostella_.
Flint, chalcedony, carnelian, onyx, sardonyx, and bloodstone, or
heliotrope, and the numerous varieties of agates, are principally
composed of quartz, with various tinging materials.
_Why is opal among the most beautiful productions of the mineral world?_
Because the colours are not occasioned by any particular tinge of the
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
is called _mina nova_.[4]
[4] The pink topaz is made from the yellow, which, when of
intense colour, is put into the bowl of a tobacco pipe, or small
crucible, covered with ashes or sand: on the application of a
low degree of heat, it changes its colour from a yellow to a
beautiful pink. It contains fluoric acid, which may be the means
of this change.--_Mawe_.
_Why is ruby of such a brilliant colour?_
Because a sixth of it is chromic acid, while other gems, as the garnet,
are coloured by oxide of iron. The most esteemed, and at the same time,
rarest colour, of the oriental ruby, is pure carmine, or blood-red of
considerable intensity, forming, when well polished, a blaze of the most
exquisite and unrivalled tint. It is, however, more or less pale, and
mixed with blue in various proportions; hence it occurs rose-red and
reddish white, crimson, peach-blossom red, and lilac blue--the latter
variety being named oriental amethyst. A ruby perfect both in colour and
transparency, is much less common than a good diamond, and when of the
weight of three or four carats, is even more valuable than that gem. The
king of Pegu, and the monarchs of Siam and Ava, monopolize the rarest
rubies; the finest in the world is in the possession of the first of
these kings: its purity has passed into a proverb, and its worth when
compared with gold, is inestimable. The Subah of the Deccan, also, is in
possession of a prodigiously fine one, a full inch in diameter. The
princes of Europe cannot boast of any of a first rate magnitude. Mr.
Mawe, from whose interesting work we abridge these particulars,
considers the oriental sapphire to rank next in value to the ruby. Among
the British crown jewels is an inestimable sapphire; it is of the purest
and deepest azure, more than two inches long, and one inch broad. The
finest ruby among these gems is more treasured for its antiquity than
intrinsic value, it being the one worn at Cressy and Agincourt, by the
Black Prince and Henry V.: this is worn on the back cross, and the
sapphire on the front, of the imperial crown upon state occasions.
_Why are garnets often found of a reddish brown tinge?_
Because of the excess of oxide of iron which they contain; a small
proportion being sufficient to colour them entirely, without injuring
their play and splendour. In fact, the perfection of all gems depends
less on the quality of their component principles, than on their
complete solution and intimate combination. The alkalized earths, as
lime, magnesia, and still better, pot-ash, seem to intervene as
solvents, for alumina, completely dissolved, acquires, as we have shown
from Klaproth, a crystallization, of which, by itself, it is not
susceptible.
The garnet is found in Bohemia, Ceylon, and other countries; but the
chief mart formerly being Sirian, the capital of Pegu, the best are
often denominated Sirian garnets. The colour most esteemed is blood or
cherry red, mixed often, however, with blue, forming tints of crimson,
purple, and reddish violet; or orange red and hyacinth brown. The Sirian
garnet is of a violet colour, which, in some rare specimens, makes it
compete with the amethyst, from which it is to be discriminated by the
disadvantage of losing its brilliancy, and acquiring an orange tint by
candlelight. Distinct from all other garnets, it preserves its colour
unmixed with the common black tinge, unassisted by foil, even when
thick. _Course garnets_ are used as emery for polishing metals, and by
lapidaries. They are found in Ireland, in Norway, and many other
countries.
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