Hi Stan,

From what I've read you want to use a very soft stone. I gather this is to allow you to sand it smooth.

Sandstone is more of a conglomerate full of hard bits like quartz, it would be worth the experiment but getting a smooth surface would be a challenge.

The purples and reds of "iron stone" sandstone would be spectacular.

Apparently you can also use soft metals, my next door neighbour is a locksmith, I might ask him for some key swarf to experiment with.