stump grinding in HPP

[buyag]

We have recently taken down a few troublesome ironwoods from our place in HPP, and plan to take down more. Now i need to get rid of the stump so I can plant something else. These stumps are approx 3 ft in diameter and located in lava rock (ripped 15 yrs ago). Anyone have experience with using a stump grinder to get rid of the stump (rental from Home Depot). Since it is located amidst lava rock, i wonder about the effectiveness of a stump grinder? Thoughts or experience?
Mahalo.
Mary