1. Peridotite: This is an ultramafic rock, meaning it is very low in silica. It is found in the Earth's mantle and is composed primarily of olivine and pyroxene minerals.

2. Dunite: This is a variety of peridotite that is almost entirely composed of olivine. It is also very low in silica.

Both of these rocks are important in the Earth's mantle and are associated with volcanic activity.