I don't know what Poly Gem is.
Tru-stone----if I get this right-----Is a name that CSUSA uses for a product line they sell.
We really misuse the name Tru-stone just because it's easy.
Tru-stone products are different in hardness----there are some that are almost impossible to turn with woodworking tools.
Richard ---who sells Tru-stone here at IAP---breaks them done into groups and helps address the hardness question of this product.
Hope that helps