There are definetly differences i the Coticule stones. Some have a distinct grain pattern in them in which a more coarse crystaline structure is seen. Most of the "Old Roc" or "exta-extra" stones as a solid, homogenous yellow with little on no grains/crystals evident.

I have gotten my best results with the more homogenous stones, other do well with the more crytstaline ones. I recently sent a very grainy looking stone to Lynn for testing and he seemed to get decent results from it.

Most natural stones have variations. The vintage Eschers I have tried all seem a little different too. Some hard, some soft with colors ranging from jet blac, to charcoal grey to a greenish/yellow tint. With any of these the underlying garnet crystals can usually be seen with a microscope.

Tony