The ancients held vases that were made of this stone in great estimation, particularly when they were of large size. Of two cups which the tyrant Nero broke into pieces in a fit of despair, when informed of the revolt that caused his destruction, one was estimated to be worth more than 600_l._ of our money. The most valuable kind of crystal that was known to the ancients was obtained from the island of Cyprus; but it was often faulty in particular parts, having flaws, cracks, and blemishes. When the crystal was used for the engraving of intaglios and cameos, the artist could sometimes conceal these defects amongst the strokes of his work; but, when it was to be formed into cups or vases, this could not be done, and for the latter purpose the purest pieces only could be employed. In the counties of Cornwall and Derby, in the neighbourhood of Bristol, and amongst the mountains of North Wales, small crystals of this kind are frequently found: these are respectively called _Cornish_, _Buxton_, _Bristol_, and _Snowdon_ diamonds. We are informed that the crevices of some parts of Mont Blanc and the Alps contain rock crystal in such abundance as to be perfectly bristled with it. Some crystals contain in their substance drops of water, or other kind of fluid; and these, as curiosities, are usually sold at a rate considerably higher than others. There are in the British Museum specimens of crystal which enclose many kinds of foreign substances, such as ironstone, needle antimony, and asbestos (136). Various means have been devised for communicating colours to rock crystal. If it be heated and plunged into a solution of indigo, or copper, it acquires a blue colour; or if into a decoction of cochineal, a red colour. A clove─brown colour may be given by exposing it to the vapour of burning wood. Artists sometimes communicate beautiful colours to rock crystals, by forming them into what are called _doublets_. Two modes of doing this are adopted. In one, a stone that is brilliant─cut at the top is hollowed underneath, filled with the colour that the stone is intended to exhibit, and then closed at the bottom by a plate of glass. If this kind of doublet be dexterously executed, the deception is not easily discovered; for the whole mass will appear of an uniform tint. The second kind of doublet is formed by cementing a coloured plate of glass on the base of a rose or brilliant─cut crystal: by this the whole stone acquires the colour of the plate. There are found in nature, many coloured kinds of crystal. These are often confounded with precious stones; and, as such, are made into female ornaments of different kinds. The following are the principal of them.