Ancient beads of orange volcanic glass in the photomicrograph above have revealed secrets of the Moon's deep interior. Produced most likely by the partial melting of the lunar mantle, and discovered by Apollo 17, these beads are unusualy rich in such volatile elements as lead, zinc, tellurium, and sulfur. This indicates not only volcanic origin but also derivation from rocks possibly as deep as 200 miles in the Moon. Similar glass beads, green in color, were discovered by Apollo 15.

From http://www.hq.nasa.gov/office/pao/History/SP-350/ch-14-4.html archive copy at the Wayback Machine

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