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Bleach will kill mold spores on contact, a 10% solution is recommended. So, a soak of 10% in a tub of water for 10-15 minutes should be enough.

The problem is bleach is corrosive and modern...

A quick soak in a bit of bleach will kill the mold, you don’t need much.

Probably how you dry them is more important to keeping the mold away. Make sure air is able to circulate around the stone....