Pirates would pick up rocks and throw them at boats, especially the British. Pirates always carried their rocks in towels for good luck and to keep them wet, which was really scary to the British... "don't 'it me wit a we' rock" they would cry before surrendering the ship. For torture, the Pirates would place wet rocks (especially dirty ones) just like this, on the Brits palms of their hands (just like your image) just before tea, then withholding wash towels, forcing them to divulge secrets.
I think the towel is the most valuable, as they didnt preserve well. Kudos to your picture of the re-enactment of the torture, I am certain the British readers are shaking at this heinous torture example.
Im thinking, an Indian pirate? This is actually the rock that most eye patches were formed over. Eye patches were traded along with glass beads. Alot of indians and pirates had one eye in those days. So my reasoning is pretty good. The weave on the towel is indicative of the indians of the carribean. Name of the tribe escapes me right now.
Now that I think about it... That rock would put some fat head's eye out in a heartbeat. Then let the person whose name escapes me right now make their own bloody eye patch! Then maybe they could actually get half of a life to go with their one eye?
Although I most highly doubt it.