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daisyruns1999
I think that it is softer than a diamond. As we all, a diamond is considered the hardest mineral and cannot be scratched by any kind penny.
jj777
This is about Mohs Scale of mineral hardness.

At a 2 it can be scratched by a penny.

Gypsum is a mineral at this hardness.

Based on this info, it is either a 2 or a 1 on the scale.

So likely Gypsum or Talc.