Chrysoprase is a nickel-rich variety of quartz with distinct green coloring.

Unlike other green minerals like emerald, which usually derive their coloring from chromium impurities, chrysoprase is colored by nickel—which also makes it slightly magnetic. Darker shades of green correspond to higher nickel content.

It is formed over millennia through a tropical weathering process called lateritization in which soluble minerals of rock are dissolved and replaced with insoluble minerals. This process was widespread until the planet cooled around 1.5 million years ago.

This mineral is cryptocrystalline, which means its crystal structure is too small to see even under magnification.

It has long been valued as a gemstone—a crystal that is cut and polished for adornment. But these gemstones are rare and fakes are common. Be wary of pieces with very uniform green color, as genuine chrysoprase is known for its lack of uniformity.