Rock-crushing scrimshaw

The Rock-crushing scrimshaw is an item that can be made with the Fletching skill at level 85 once all four scroll fragments of Rock Crushing have been retrieved from the Player-owned port.

When equipped, it offers a 15% chance to crush the ore (losing it in the process) in exchange for increased experience while Mining.

The crushed ore appears to give 50% bonus experience, which means 15% chance to give 50% bonus experience. This works out at about 7.5% bonus experience. Rates at high mining are approximately 90k/h so 3 hours of bonus exp gives a total of 3 x 0.075 x 90,000 = 20,250 bonus experience total, although you will probably get a bit more than this due to the decrease in banking needed.