Geology 101: At the Museum and In the Field main content.

Geology 101: At the Museum and In the Field

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There are three kinds of fundamental rock: igneous, sedimentary, and metamorphic. While rare, gems can form or be found in various environments within all of these rock types, via different processes. 

Igneous rocks form when molten rock (magma or lava) cools and solidifies.

Igneous rocks
Igneous rocks in the Hall of Planet Earth

Sedimentary rocks originate when particles settle out of water or air, or by precipitation of minerals from water. They accumulate in layers.

Sedimentary rocks
Sedimentary rocks in the Hall of Planet Earth

Metamorphic rocks result when existing rocks are changed by heat, pressure, or reactive fluids, such as hot, mineral-laden water.

metamorphic rocks
Metamorphic rock in the Hall of Planet Earth 

While almost all gems are formed below Earth’s surface, some are unearthed through mining, while others are exposed through Earth processes such as volcanoes, large-scale uplift, or faulting and, of course, weathering.

Gottesman Hall of Planet Earth
Gottesman Hall of Planet Earth
AMNH / R. Mickens

Interested in hunting for minerals near New York City this summer?

It’s illegal to collect on private lands without permission, but Curator Jim Webster recommends a few sites where collecting or prospecting is encouraged.

Sterling Hill Mining Museum, Ogdensberg, NJ: Just an hour or so from New York City, visitors ages 7 and up can collect zinc-ore mineral specimens, which are often fluorescent. Wear appropriate shoes and bring safety glasses as well as your own hammers, ultraviolet lamps, and carrying bags. Tools are also available for purchase. For more information, including fees, visit

Herkimer “Diamonds,” Upstate New York: Hunt for unusually clear, diamond-like quartz crystals near Albany, New York. Two commercial mines offer prospecting from April through October. Plan to bring appropriate footwear and safety glasses as well as tools. For more information about admission and fees, visit and

Bear Mountain State Park, Bear Mountain, NY: While you can’t collect here, this popular park was formerly mined for iron ore and offers hikes that take you by historic mine dumps, mining roads, and prospect pits. For more information, visit

Learn more about different types of rocks at the Gottesman Hall of Planet Earth. For more about minerals and gems, visit the Guggenheim Hall of Minerals and the Morgan Memorial Hall of Gems.

This story is adapted from an article in the Summer 2014 issue of Rotunda, the Member magazine.