USGS Geology in the Parks
sand beaches are very rare, but two popular ones lie within the
US: one on Hawai'i, one in the territory of Guam. Green sand is
composed primarily of olivine crystals which erode out of basalt
(lava) flows. The crystals are heavier than most sand types on the
beach and remain behind when lighter sand grains are washed away
by strong wave activity.
Olivine is a silicate mineral that contains iron and magnesium.
These density the of the mineral, a two elements, make it heavy.
Olivines form at high temperatures and are green and glassy. They
are common in basalts, especially those found of the ocean-floor,
and in ultramafic
rocks. Rocks made up entirely of olivine, such as the example
at right, are called dunite, and gem-quality olivines are called