We will now make some observations in reference to precious stones in general, following therein the opinions that have been expressed by various authors. Stones with a level surface are preferred to those which are concave or protuberant on the face. An oblong shape is the one that is most approved of, and, next to that, the lenticular[1] form, as it is called. After this, the stone with a plane surface and circular is admired, those which are angular being held in the least esteem. There is considerable difficulty in distinguishing genuine stones from false; the more so, as there has been discovered a method of transforming genuine stones of one kind into false stones of another.[2] Sardonyx, for example, is imitated by cementing together three other precious stones, in such a way that no skill can detect the fraud; a black stone being used for the purpose, a white stone, and one of a vermilion[3] colour, each of them, in its own way, a stone of high repute. Nay, even more than this, there are books in existence, the authors of which I forbear to name,[4] which give instructions how to stain crystal in such a way as to imitate smaragdus and other transparent stones, how to make sardonyx of sarda, and other gems in a similar manner. Indeed, there is no kind of fraud practised, by which larger profits are made.