Where lithospheric plates pull apart, along the midocean ridges, the release of pressure on the peridotite mantle allows it to partially melt. That melted portion, richer in silicon and aluminum, rises to the surface as basalt.
This peridotite boulder is partially altered to serpentine minerals, but it has visible grains of pyroxene sparkling in it as well as serpentine veins. Most peridotite is metamorphosed into serpentinite during the processes of plate tectonics, but sometimes it survives to appear in subduction-zone rocks like the rocks of Shell Beach, California. See more examples of peridotite in the Peridotite Gallery.