Tenergy 64 fx.
Brittle as cookies / biscuits!
If you had a sheet of T64fx and the topsheet was any different than any other Tenergy you got a bad sheet or a counterfeit sheet. The rubber compound (formula) in the T05, T05fx, T80, T80fx, T64, T64fx is all exactly the same. The only difference is the pimple structure. If it was the sponge, same: the sponge on all those FX models are the same. The sponge on all the regular models are the same. The difference between regular and FX is the hardness of the sponge. So T05 and T05fx have the same exact topsheet and the same exact pimple structure on the topsheet: only the sponge is different (softer on FX model). Same with T80 and T80fx and T64 and T64fx. The rubber compound for the topsheet, for all of them is the same. The sponge for all the regular Tenergies are the same. The sponge for all the FX models are the same. The only difference is sponge hardness between regular and FX and pimple structure between 05, 80 and 64 and those numbers indicate something about the pimple structure.
So, if it is the topsheet, either all the Tenergies are brittle as a biscuit or you got a bad one. If it is the sponge, either all Tenergy FXs or you got a bad one.
But it is the first time I have ever heard anyone describe any Tenergy rubber as brittle as a biscuit.

It sounds entertaining. But....