Peridotite in volcanic basalt with vivid color

Locale: Arizona

Approx dimensions: 2.75 x 1.75 x 1.25"

Approx weight: 158g

While this material is commonly called peridot in (or ‘on’) basalt, it is actually more complex. It would be more accurately called a peridotite xenolith in basalt. Peridot is the gem-quality version of the mineral olivine, which is common in our Earth’s upper mantle—and even in space. In comparison, peridotite is a combination of several types of minerals that primarily features olivine, but can also contain spinel, garnet, diopside, and other igneous minerals. This medium-quality material from Arizona has peridotite containing olivine which is light yellow-green and gemmy when higher quality, but a duller desaturated brown-green when lower quality—as seen on one end of the piece. This chunk has a large patch of gemmy quality crystals, where you can see irregular crystals of chrome diopside (vivid green), and spinel (deep forest green).

Xenolith means there are pockets of ‘foreign’ material, peridotite, included within the igneous rock, basalt. This is what's called vesicular basalt, referring to the classic 'volcanic rock' style of basalt full of cavities left behind by gas bubbles. The peridotite would have formed in the upper mantle but pieces of it became entrapped in magma during the eruption that transported it to the earth’s surface. This specimen has an interesting feature: a thin elongated bubble that goes all the way through the piece, so you can see through it.

** Brittle warning! This material has the tendency to be crumbly, shedding pieces when handled. I find it to be a beautiful accent rock for my garden and do keep some pieces outdoors despite this risk. Just keep it out of direct sunlight and the elements!