Rock butter (also known as stone butter) is a soft mineral substance found oozing from alum slates. It consists of native alum mixed with clay and oxide of iron, usually in soft masses of a yellowish white colour with translucent edges, occurring in cavities and fissures in argillaceous slate.
Rock butter (also known as stone butter) is a soft mineral substance found oozing from alum slates. It consists of native alum mixed with clay and oxide of iron, usually in soft masses of a yellowish white colour with translucent edges, occurring in cavities and fissures in argillaceous slate.
Source: Wikipedia "Rock butter" · CC BY-SA 4.0
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