I have just finished sitting down and getting straight the different types of ultramafic rocks, as crystallized in two triangular (ternary) diagrams. Ultramafics are what make up most of the upper mantle; they're dark mixtures of olivine, pyroxene and amphibole minerals that are rarely seen at the Earth's surface, usually in ophiolites. On the first diagram, ultramafics fall into four high-level rock types, of which twoperidotite and pyroxenitecan be broken down into several more rock types. The second diagram gives those along with a bit of the underlying geologic significance.
Peridotite Geology Guide photo