Dunite is a deep-seated, heavy rock that consists almost entirely of the mineral olivine. Big bodies of dunite are quite rare. Your basic rock-shop specimens are usually dunite xenoliths, little lumps carried up within a basalt lava flow from Arizona. That's where this specimen is from. See it here with its fellow igneous rock types.
Dunite xenolith — Geology Guide photo
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The type locality for dunite is Dun Mountain, near Nelson in the South Island of New Zealand. Dun mountain is only half of the massif, though. The other half, Red Mountain, is many hundreds of kilometres south, thanks to the dextral slip of the alpine fault. The matching of these two halves, and the rest of the offset sections of the South Island’s ophiolite belt were important in establishing that the massive fault line existed.
It must be an impressive place. (Thanks for the name correction.)
Andrew,
Looks like I have one up on you. I did visit a dunite quarry near Spruce Pine NC. What was interesting to me was not only all that Dunite, but seeing how green rock weathered into yellow-brown soil. Because of the high melting point, dunite/olivine is used in casting metals.
I have seen dunite from Oregon so there should be a deposit on your side of the continent.
PRN