Can anyone tell me what these are?

I bought some of these balls quite some time ago and I can't remember what they are called. Can anyone help in identifying them?

Many thanks, Michael

Attachment 10918

It's not polymer clay, is it? The texture is identical to some polymer discs I have.

Many thanks for your prompt reply, Sheen.

The mystery has been solved. They are dark blue rough weathered agates. An important lesson for me here - don't throw receipts away.

Thanks again, Michael