Magnesium-rich spinel and olivine found in the central peaks of lunar craters, ... On Earth, spinel forms under high temperatures and pressures not seen near the Moon's surface.

Peridot, spinel and sardonyx are August's three birthstones. Need a break? Play the USA TODAY Daily Crossword Puzzle. Peridot is a yellow-green stone that comes from the mineral olivine.

Unusual minerals like spinel and olivine found in many lunar craters, but rarely on the Moon's surface, were therefore attributed to the excavation of sub-surface lunar layers by asteroid hits.

"At these depths, olivine should transform to the stable phase, spinel," Green said. "The very cold temperatures deep in the downgoing slabs inhibit this transformation.

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