Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine.
I wanted to take this sentence apart to make it understandable to everyone but then I discovered that I started to write lengthy paragraphs about ultramafic rocks in general, not about dunite in particular. So, I will leave that material for a separate post now. Very shortly: ‘ultramafic’ means that mafic minerals form more than 90% on the rocks composition. Most common mafic minerals in ultramafic rocks are definitely pyroxenes and olivine (if hornblende is present it is added to pyroxenes). Rocks that contain more than 40% olivine are peridotites. Note that this 40% means 40% of olivine-pyroxene(hornblende) pair, all other minerals are excluded in current classification scheme. Peridotite that contains more than 90% olivine have a special name, they are called dunite (named in 1864 after Dun mountain in New Zealand).
‘Plutonic’ means that the rock is not volcanic, it didn’t form at or near the surface. In the case of dunite the formation place was probably very deep in the mantle. That’s why it is so rare on the surface. Dunite is rare but it is pretty. However, its beauty is not the reason to reserve a separate rock name for it. It is an important rock type because it is probably very common in the mantle.
Dunite is mostly composed of olivine which is a bright green mineral. Fresh dunite is green as well. However, olivine readily alters and loses its bright green color pretty quickly. Chances are very high that on the way up in the crust olivine grains lost some of its brightness. Hence, many dunite samples look dull yellow, not green anymore. Dunite usually contains chromite (Mg-bearing spinel group mineral). However, if the most common spinel mineral is magnetite, dunite is named olivinite instead.
Magnesium is a very common chemical element in the mantle. Therefore, we should expect to see lots of unusual mineral varieties in dunite. I already mentioned chromite but Mg-bearing garnet pyrope is quite common as well. If the dunite sample contains significant amount of garnet, then it should be added to its name. The rock sample below is a garnet peridotite because it contains one very large and several smaller purplish pyrope crystals. It is probably not true dunite because it contains more than 10% chromian diopside (bright green mineral). It should be named either wehrlite (if there is almost no orthopyroxene) or lherzolite (more than 10% orthopyroxene).