Set with touchstone
It is called touchstone to the stone that serves to know the purity of the material with which the piece is made, it is generally used for gold, but also for silver.
The touchstone is a hard, dark-colored stone composed of a mixture of amorphous quartz with alumina, lime, iron oxide, coal and other fine-grained substances that can not be attacked by acids. These qualities make it suitable for the purity test of metals.
When you want to try a gold jewelry, rub lightly on this stone until it has produced a trace and then pour a drop of nitric acid. If the jewelry is made of pure gold, the print does not suffer any alteration but if it is not pure, the color of the print indicates the proportion in which it is alloyed.
In ancient Greece and in the Indus Valley civilization it was used to know the purity of gold. The stone was scratched with pieces of various alloys of gold and copper and the color of the scratch depended on the amount of gold that it had, so that when it was necessary to know the content of precious metal of some unknown piece, it was enough to scratch the stone with it and compare the color of the scratch with those produced by the other pieces.
This article contains material from the encyclopedic popular illustrated dictionary Salvat from the years 1906 to 1914 that is in the public domain.
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