The Superior rock-crushing scrimshaw is a pocket slot item that can be made at your Player-owned port workshop at level 85 Fletching from 10 Ancient bones after all four scroll pieces have been obtained. When activated the scrimshaw will last for four hours before it degrades to dust.
When equipped, it offers a 20% chance to crush the ore (losing it in the process) in exchange for increased Mining experience. This bonus experience stacks with the Golden Mining Suit.
The crushed ore appears to give 38% bonus experience, which means 20% chance to give 38% bonus experience. This works out at about 7.6% bonus experience