THE DYEING OF PRECIOUS STONES.

The possibility of giving artificial colours to Precious Stones was not unknown to the Romans. Pliny relates that recipes ware offered for sale which professed to turn Rock-Crystals into Emeralds and other transparent gems ; that in India many Precious Stones were produced by dyeing Rock-Crystal, and that the Ethiopians deposited the pale Carbuncle in vinegar for fourteen days, when it was alleged that it would shine brilliantly for a similar number of months.
Respecting the artificial colouring of certain Agates, Pliny says that in his day more of these stones were probably coloured artificially than naturally : and that in Arabia the Agate-nodules, if cooked seven days and seven nights in honey, will, when prepared by the artist, present veins, stripes and spots, which increase their effectiveness as ornaments.

This notion of honey purifying the Agate seems to be the foundation of the following beautiful idea : " All kinds of Precious Stones, cast into honey, become more brilliant thereby, each one according to its colour, and all persons become more acceptable in their vocation, when they join devotion with it : household cares are thereby rendered tranquil, the love of husband and wife more sincere, the service of the prince more faithful, and all kinds of business more easy and pleasant."—Extract from the Introduction to " The Devout Life" by S. Francis de Sales. Chap. III., par. 13. 1708.

THE BURNING AND COLOURING OF PRECIOUS STONES.

CERTAIN kinds of Precious Stones are often burnt or subjected to a high temperature, the heat exercising a very peculiar influence upon many stones, and in some cases modifying or utterly changing their colour.

Thus, the Oriental Carnelian owes its beautiful tint to artificial exposure to heat. The Pink Brazilian Topaz, too, derives its remarkable colour from burning.

One way of burning Precious Stones is to roll them up in a piece of sponge or tinder, and set fire to the enveloping material. Another method is to place them in a crucible, with either unslaked lime or iron-filings, and heat them until they are quite clear. Occasionally where a faulty stone with dark spots is burnt with sand and iron-filings, the spots are removed and the colour equalized ; but the process requires great care.

Rubies are occasionally infected with white spots which can be removed by burning. Many coloured Jar-goons have their tints more or less completely discharged on exposure to a high temperature. Smoky Rock-Crystal also, carefully heated in a crucible with lime, sand, or charcoal, will usually come out perfectly clear. The discharge of colour is evidently due to the decomposition of the organic matter, with which the stone was tinted.

DEFINITION OF THE TERM PRECIOUS STONE OR GEM.

AMONG the infinitely diversified products of Inorganic Nature, there are certain mineral substances which form a small class by themselves—standing apart from all others by the possession of such exceptional characters that they have always attracted the attention of persons endowed with taste and refinement. These minerals, distinguished as Precious Stones, are nine in number, namely :

—the Diamond. Chrysoberyl

Ruby. (True Cat's Eye).

Sapphire. Alexandrite.

Spinel. Opal.

Emerald. Turquoise.

The characters which have commended such stones in all ages, for purposes of personal ornament, are chiefly their brilliancy and colour, their durability and rarity. It is not sufficient, however, that a stone should possess only one of these characteristics, The mineralogist is familiar with many stones that are exquisite in colour, yet far too soft to be used for the'"practical purpose of decoration ; on the other hand, there may be stones of exceeding hardness