How Does Sand Get Its Color?

Sand’s color is derived from its mineralogy, or the physical structure of the crystals that populate the sand.

Sabrina Stierwalt, PhD
4-minute read
Episode #271

What Causes Sand to Be Purple?

The Pfeiffer Beach tucked between the ocean and the Los Padres National Forest in Big Sur, California, boasts unusual purple-hued sand. The sand gets its color from a mineral called manganese garnet which is found in the hills that surround the beach. Rain in the area causes erosion which washes the manganese garnet down toward the ocean. The beach is not uniformly purple and the erosion runoff can create swirled patterns of colored sand. To see the beach at its most purple, visit just after a winter storm has sped up the erosion process.

Large swatches of purple sand are also found on Plum Island Beach off the eastern coast of the United States. On Plum Island, home to seals, raptors, and snowy owls, the purple sand results from a mixture of fine grain pink sand with darker grains. The pink sand crystals are mostly made of almandine-pyrope garnet, a common mineral derived from metamorphic rocks in the area. Some of the darker grains even include green epidote, another mineral common in metamorphic rock.

What Causes Sand to Be Black?

Black sand beaches, like Punalu’u Beach in Hawaii, get their color from basalt, a common igneous rock that forms as lava cools. In Hawaii, underwater volcanic vents spew magma which cools rapidly when it meets the ocean water. The resulting chunks of basalt then wash up on shore to cover the beach in grains of black sand.

If you are looking for more than one color in your beach sand, the Rainbow Beach in Queensland, Australia.

Past volcanic activity has also created the Black Sand Beach in Vik, Iceland, and the Muriwai Black Sand Beach in Auckland, New Zealand, thanks to a mixture of basalt deposits, iron, titanium, and other volcanic material. The black sand on Playa Negra in Puerto Viejo, Costa Rica, also has the fun, added quality of being magnetic thanks to its iron content.

What Causes Sand to Be Green?

The mineral olivine is responsible for the beautiful but rare green sand beaches, like Papakolea Beach on the big island of Hawaii. Olivine is another mineral that, like basalt, forms as lava cools. The olivine at Papakolea was deposited there by the Pu’u Mahana cinder cone, part of a now-dormant volcano whose magma was rich in olivine. The clash of hot lava and cold ocean water also produce the olivine-rich green sand beaches in Kourou, French Guiana.

If you are looking for more than one color in your beach sand, the Rainbow Beach in Queensland, Australia, offers sand with what some say are more than 70 different colors derived from erosion of nearby, multi-colored cliffs. And not all sand colors are natural in origin. Two beaches known as Glass Beach, one along the southern shore of Kauai and the other near Fort Bragg, California, also boast a rainbow of colors but are entirely human-made. The multi-colored sea glass that makes up the “sand” on both beaches may be gorgeous to look at but comes from glass pulled out to sea from industrial-sized garbage dumps nearby and smoothed by ocean waves over decades.

Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image courtesy of shutterstock.


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.