The red-orange glow of a lava flow rapidly turns to shades of gray as the lava cools to form solid rock. Rich in magnesium and iron, lava flows of Hawaiian Island volcanoes often produce black basalt speckled with green crystals. Not to be mistaken for emeralds, the crystals belong to a series of magnesium-iron silicate minerals collectively known as olivine, including fayalite and forsterite. Although most olivine remains trapped within the volcanic rock from which it arose, free crystals can be found in abundance where lava flows are exposed to the erosive forces of the ocean. On a remote coastline near the southernmost point of Hawaii's Big Island, such forces continuously acting on the ancient Pu'u Mahana cinder cone have colored the sand of an entire beach green with tiny olivine crystals. Known locally as Papakōlea Beach, it's no mystery why this popular tourist destination is better known as Hawaii's Green Sand Beach.