Gems and precious stones are something I knew nothing about. So I’ve been reading Victoria Finlay’s wonderful book Jewels.
One thing that caught my eye was the Mohs Scale. In 1802 a Viennese banker bought eleven of the best mineral collections in Europe. Like lots of collectors, he wanted the stones catalogued and classified. To do so, he hired Friedrich Mohs, a young mineralogist. One of Mohs’ big problems was measuring the hardness of stones, and how to quantify how much harder a lump of topaz is than a lump of gold.
Rather than go for something absolute, he opted for a comparative scale.
So he chose ten minerals ‘of which every preceding one is scratched by that which follows it’, and gave them each a number according to his new scratching order. The softest was talc, which scored one, while the hardest was diamond, which therefore got ten. Everything else came out somewhere in the middle … He acknowledged that the values were arbitrary … but the system was at least easy to use. You can do it at home: anything that can be scratched by a fingernail is below two; anything that takes a mark from a pocket-knife blade is below five; anything that can scratch quartz is above seven.
Apparently seems that
the materials engineer and metallurgist find little use for the Mohs scale
Fair enough. But what I liked was the huge power of a rough heuristic for most people.