serpentine

A group of green, slippery-feeling metamorphic rocks formed in subduction zones, and the unusual soils they produce — low in calcium, high in magnesium and heavy metals, toxic to most plants. Serpentine soils are ecological islands: the generalists can't grow there, so the specialists have the ground to themselves. Some of California's rarest wildflowers exist only on serpentine. The bald patches, the stunted trees, the sudden shift in vegetation on a hillside — often that's serpentine underneath.