PT, thanks, I appreciate you taking the time to do that.
According to my calculations, the density is .056 lb/inch^3. I don't have the specs for fibrament, but cordierite runs in the .09 range, so I think it's safe to assume that this isn't cordierite.
Strictly speaking, it can't be a pure Fibrament analog without the reinforcing fibers, but it's definitely cast cement rather than kiln fired cordierite. This is basically the same material as the cheap walmart stones, just thicker. I might recommend eventually replacing it with cordierite, if it weren't for the fact that the low density/low conductivity is an important part of maintaining the proper heat balance, so, when the stone goes, it will need to be replaced with either same stone or something equally as light/fragile.
I'm curious, are there any instructions for curing? Even if there aren't, just to be safe, I'd probably cure this stone (start on lowest oven temp and then increase by 100 every hour).