i just seen them on ebay.com and they are called canyon rocks. $79 or so for a lot of 8 they def wud look awsome in a tank, jus look at that link i put up worth the expense. i think. but im in ireland so the rocks being in the US and in dollars aint no use to me.lol. ill have to check round LFS
that kind of rock is a canyon type rock but it is actually made from wind being blown through it. Or atleast its naturally occuring type is. In nature it is called arch rock. such as the rocks in brice canyon.