'ROTTEN STONE' - Brazil

This is a laminated, very fine-grained, lightweight and porous rock. It is from an old museum collection and has an antique label attached giving the name 'rotten stone'. 

Rotten stone usually consists of weathered limestone mixed with diatomaceous or crystalline silica. It has similar applications to pumice, but it is generally sold as a fine powder and used for polishing.

  
 A curious specimen which needs more research.

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