Peridotite mantle xenolith in basalt lava from Arizona, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

The greenish-colored mass is a sample of mantle rock. It's a xenolith nodule in basalt lava that was erupted from a volcano. The rock was plucked from deep wall rocks during the ascent of magma during the Late Pleistocene. The xenolith itself is peridotite, an olivine-rich, ultramafic, intrusive igneous rock. Peridotite has a phaneritic texture (= coarsely-crystalline) and formed by slow cooling.