140	Gem Trader
	bers of the family of crystalline silicas are the rock crystal and the citrine. You will remember that the non-crystalline members of the family of silica are the opal and the chalcedony, which shows that very minute differences in the molecular structure of a substance make all the dif­ference to its appearance and its value in the market. Of course, if the eyes of Cleopatra had been just the merest trifle crossed, that would have made all the differ­ence in the world, too. Cairngorms are found plentifully in Scotland and Corn­wall. They are therefore, unlike many gems, native gems of Britain. They occur much to the benefit of local lapi­daries and the delight of tourists, who are ever on the lookout for something that can be picked up "cheap". Many a lady wears a cairngorm proudly because it re­minds her of her honeymoon or some other sentimental occasion. It is quite pretty enough, too, to sustain an affection of that kind.