probably calf skin leather, but hard to tell from the angle of the pic. i've also seen that design in croc or gator, can't remember which, but can send you a link if interested.

check out this link for another pic/look, but i have to warn you i just did a google search for double ridged watch straps and this site came up and had a pic of a strap, similar to the one you are asking about. in other words i have no info or feedback from or about this site, only using as a pic ref. look for the pic with double ridge.

j.h.