Crystals and Pliny the Elder
This excerpt is from Book XXXVII, Chapter 10 of a book by Pliny the Elder called the "Natural History". This chapter is called "Luxury Displayed in the Use of Crystal and Remedies Derived from Crystal". Of course, anything that was shiny or translucent would be of interest as a decoration to the ancients. Crystals were thought of somewhat as baubles that could be used in Jewelry. However, we are dealing with the Romans here. And in this case we see the practical Roman mind applied to crystals in general.
The largest block of crystal that has ever been beheld by us, is the one that was consecrated by Julia Augusta in the Capitol, and which weighed about one hundred and fifty pounds. Xenocrates speaks of having seen a vase of crystal, which held one amphora, and we find other writers mentioning a vessel from India which held four sextarii. For my own part, I can positively say, that there is crystal amid the crags of the Alps, so difficult of access, that it is usually found necessary to be suspended by ropes in order to extract it. Persons who are experienced in the matter detect its presence by certain signs and indications.
Crystal is subject to numerous defects, sometimes presenting a rough, solder-like, substance, or else clouded by spots upon it; while occasionally it contains some hidden humour within, or is traversed by hard and brittle knurrs, which are known as "salt grains." Some crystal, too, has a red rust upon it, while, in other instances, it contains filaments that look like flaws, a defect which artists conceal by engraving it. But where crystals are entirely free from defect, they are preferred uncut; in which case, they are known as "acenteta," and have the colour, not of foam, but of limpid water. In the last place, the weight of crystals is a point which is taken into consideration.
I find it stated by medical men that the very best cautery for the human body is a ball of crystal acted upon by the rays of the sun. This substance, too, has been made the object of a mania; for, not many years ago, a mistress of a family, who was by no means very rich, gave one hundred and fifty thousand sesterces for a single basin made of crystal. Nero, on receiving tidings that all was lost, in the excess of his fury, dashed two cups of crystal to pieces; this being his last act of vengeance upon his fellow-creatures, preventing any one from ever drinking again from these vessels. Crystal, when broken, cannot by any possibility be mended. Vessels in glass have been brought to a marvellous degree of resemblance to crystal; and yet, wonderful to say, they have only tended to enhance the value of crystal, and in no way to depreciate it.
Possibly the most interesting section in this extended quote is the comment that crystal, with the aid of the sun, was used to cauterize wounds. This shows that the ancients at least knew that certain crystal shapes would concentrate the sun's rays to the point of burning the skin. It would also later have implications when aplied to the microscope as the magnifying properties of crystal came to be recognized and exploited.