glass-mixtures, though they do not detract from the physical properties of the glass. The higher powers of the same instrument will almost always define the junction and the layer or layers of cement, no matter how delicate a film may have been used. Any one of these tests is sufficient to isolate a false stone. Some of the softer genuine stones may be fused together with splinters, dust, and cuttings of the same stones, and of this product is formed a larger stone, which, though manufactured, is essentially perfectly real, possessing exactly the same properties as a naturally formed stone. Many such stones are obtained as large as an ordinary pin's head, and are much used commercially for cluster-work in rings, brooches, for watch-jewels, scarf-pins, and the like, and are capable of being cut and polished exactly like an original stone. This is a means of using up to great advantage the lapidary's dust, and though these products are real stones, perhaps a little more enriched in colour chemically, they are much cheaper than a natural stone of the same size and weight. Some spurious stones have their colour improved by heat, by being tinged on the outside, by being tinted throughout with a fixed colour and placed in a clear setting; others, again, have a setting of a different hue, so that the reflection of this shall give additional colour and fire to the stone. For instance, glass diamonds are often set with the whole of the portion below the girdle hidden, this part of the stone being silvered like a mirror. Others are set open, being held at the girdle only, the portion covered by the setting being silvered. Other glass imitations, such as the opal, have a tolerably good representation of the "fiery" opal given to them by the admixture, in the glass, of a little oxide of tin, which makes it somewhat opalescent, and in the setting is placed a backing of red, gold, copper, or fiery-coloured tinsel, whilst the glass itself, at the back, is painted very thinly with a paint composed of well washed and dried fish-scales, reduced to an impalpable powder, mixed with a little pure, refined mastic, or other colourless varnish. This gives a good imitation of phosphorescence, as well as a slight pearliness, whilst the tinsel, seen through the paint and the curious milkiness of the glass, gives good "fire." A knowledge of the colours natural to precious stones and to jewels generally is of great service in their rough classification for testing, even though some stones are found in a variety of colours. An alphabetical list of the most useful is here appended, together with their average specific gravities and hardness. (See also Chapter VII. on "Hardness," and Chapter VIII. on "Specific Gravity.") WHITE OR COLOURLESS STONES. _Hardness._ _Specific Gravity._ (See Chapter VII.) (See Chapter VIII.)