It looks to be plasticine modeling clay. I use it occasionally because it's sulfur free, since sulfur affects some platinum cure silicon moldmaking processes.
Thanks
Pryo's right, It's plasticine clay. I like it because you can have a 25 lb brick and it goes a looong way. Plus it's a sulfur-free brand so I can heat it in my oven without it releasing toxic fumes (always a good thing, no? ).
some of the depths of your crevaces are off. Many things you thing should be pits are aftually just etched lines. look at some CLOSE UP pictures. they really helped me