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 [1] with translucent edges, [2] : 307 occurring in cavities and fissures in argillaceous slate.
This substance hardens when exposed to air, but softens if the air is humid enough. [3] : 28 The texture is sometimes described as "greasy". [2] : 307
It was referred to as "rock butter" in English as early as 1816. [2] : 306–7
It has been recorded in various locations around the world, including Paisley, Scotland; Bornholm, Denmark; Bad Muskau in Germany; and along the Yenisey in Siberia. [2] : 307 An 1837 article noted that at the "Kiffhäusen" stone quarries in Germany, the workmen called it steinbutter and ate it spread on bread. [4] : 37
There have been reports of people eating rock butter in various parts of the world, including Siberia, [3] : 28 Germany, and Austria.
