Kosketinsä

Kosketinsä is a Finnish term that translates to "touchstone" in English. It refers to a type of hard, fine-grained rock, typically a dark color like black or dark gray, that is used to test the purity of precious metals, especially gold and silver. The process of using a kosketinsä involves rubbing the metal against the stone to create a streak. The color of this streak is then compared to streaks made by known standards of pure gold or silver, or alloys of known fineness.

The effectiveness of a kosketinsä relies on its fine texture, which allows for a clear and distinct streak to be produced. The stone itself must be non-reactive with the metals being tested to ensure an accurate result. Historically, touchstones have been crucial tools for assayers, jewelers, and merchants in verifying the value of gold and silver objects and coins. Different types of kosketinsä exist, with variations in composition and texture affecting their suitability for testing different precious metals and alloys. The practice of using a touchstone is a simple yet effective method of preliminary metal analysis, predating more sophisticated scientific techniques.