You know I have a piece similar to yours. It is not as rounded as yours but does gave that reddish tone to it, and it does not sink! The guy I bought it from said he had it in his tank for 2 years weighed down and also floating and it never sunk. Heres a couple pics, curious myself to see what type of pieces you and I have.
55 Gallon Planted
3 GBR, 2 Bolivian Rams, 1 Juvenile Super Red BN Plecos, 9 Oto Catfish, 9 Bloodfin Tetras
Rotala Indica, Cardamine, Crypt Spiralis, Pygmy Chain Swords, Dwarf Sag, Lutea Crypt, Wendtti Bronze and Red, and Floating Frogbit