as well as EVERY SATURDAY in AUGUST
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Generally, a black siliceous stone (usually basanite) related to flint that is used to test the purity of gold and silver by the streak left on the stone when rubbed by the metal. In recent years some other material is sometimes used for the purpose, the requirements being that it is black, harder than the metal to be tested, and finely grained, e.g. unglazed black basaltes made by Wedgwood.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson