The turquoise being an opaque stone, it would be useless to cut facets upon it, as these would not reflect light in the same way as when fashioned upon a transparent gem like a diamond or a sapphire. There are three ways of cutting the turquoise, all much in the same style—the flat or slightly convex form, the truncated cone, and the tallow drop or _en cabochon_. The higher the conical and convex surfaces in the two latter, the more the turquoises are prized. None but a fine deep-coloured stone can be advantageously cut into a conical shape, since one of pale colour would appear almost white at the apex. Turquoises are cut by the hand on wheels made of a composition of emery and gum. They are afterwards polished by being rubbed on a fine-grained sandstone, and then on a piece of soft leather with turquoise dust. Of the few mines which yield good turquoises, one or two are dangerous, on account of the loose rubbish they contain. The one from which the best of all are obtained yields very few. Some mines contain stones which look well at first, but soon change their colour and fade. Mr Schindler gives an instance of a recently found turquoise, as large as a walnut and of fine colour, being presented to His Majesty the Shah, which he had for only two days, when it became green and whitish, and therefore of no value. Throughout Europe, there has been a great fall in the price of this gem within the last few years, and it would seem that this is owing to the fact that large quantities of stones which appeared to be of fine quality, but were really of fugitive colour, had been disposed of not long ago at good prices. Up to the time that they were sold, their colour had been preserved by keeping them damp; but when taken out of their moist packing, they slowly became white. It need hardly be said that the colour of most precious stones is very permanent. There is, however, a variety of opal occurring in Mexico which is very beautiful when first found; but after a brief time it entirely loses its bright play of colours. Both the turquoise and the opal are peculiar in containing a considerable amount of water in their composition.