A type of metamorphic rock or mineral, specifically a stone that forms under heat and pressure and is used in certain industrial applications.
From German 'Fett' (fat/greasy) plus 'Stein' (stone), referring to the greasy or waxy appearance of this mineral. The term was adopted into English from geological German terminology.
Fettstein looks almost soapy to the touch because of its mineral composition—it's called 'greasy stone' in German because geologists thought it looked like someone had rubbed fat on it!
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