Stone inlay question

[Gavin Nugent]

I have an old Madura keris hilt in ivory that appears to have once had a cabachon inlayed to the centre of the stylised sun motif on its back.
The size would have been just a fraction under 8mm x 5mm and flat backed.

My questions are;
1/ Would this have been a stone or something else?
2/ If a stone, what type would it be?
3/ If it wasn't a stone, what would it be?

The recess retains old black pitch.

Thanks

Gavin