PRECIOUS STONES.
(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]
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.