Polished precious and decorative stones.
From the earliest times, precious and decorative stones have been ground, smoothed, which added shine to them, brilliance and led to a more complete presentation of the color. For several thousand years, initially concave has been carved in precious and decorative stones, and then convex reliefs. Seals for marking goods, signet rings for sealing royal acts and correspondence, and even worn as cameo decorations – portraits of famous people, hunting scenes, animal figures etc.. So it has long been known, that some stones are soft, easily workable, others very hard, harder than glass and all metals. So hard, that they can only be ground with the hardest dust among all known bodies – diamond.
It was one of the pieces of information about the physical characteristics of gemstones. The second information could be obtained even before the day before, in which Archimedes jumped out of the tub shouting "Heureka!”. It was enough to measure the density (or as written in physics textbooks - specific gravity) all gemstones.
Using both, the English physicist and chemist Robert Boyle announced in 1762 years of presumption, that ruby and sapphire are the same stones, because they have the same hardness and identical density, and they differ only in color. If it turns out to be true, the prevailing basis for the classification of gemstones would be fundamentally false. But how to prove it?
In the 20th century, we already know how. By the method of chemical analysis; by checking, what elements the ruby is made of, and from which sapphire. Primitive chemical analysis was already used in antiquity and the Middle Ages. This was necessary to obtain metals from newly discovered ores and to prepare dyes for decorating pottery. Many methods of recognizing ores are described, for example,. in a famous book by a German teacher of Greek and Old Hebrew, miner, geologist, a metallurgist and a doctor at the same time (such versatile people were in those days), Georgiusa Agricoli, issued in 1556 of the year Fri. "It's metallic” (The thing about metals). The methods of chemical examination of metal ores described by him survived until the mid-18th century, but they were too imprecise to distinguish between ruby and sapphire.