Answer: In my experience there is no real difference in the performance of the stones although white rock can create glare and is heavier to work with. Smaller pieces of the lava rock may shift a bit along the edges in heavy rains. They will both hold an excessive amount of heat and will reflect it back onto the plants; they will not break down so will not help improve the soil over time. However, they will both help keep the soil somewhat moister than it would be without mulch, rocks stay in place well and they will not attract termites as a wood mulch could. Generally it becomes a matter of personal preference.
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