A stone diary

Visitors entering the Museum's 77th Street Grand Gallery may find themselves doing a double-take at what appears to be a cross-section of a huge creme de menthe parfait. But the recently installed polished jade slab is actually a much weightier treat, offering a rare object lesson in mineral formation and plate tectonics.

Like a geologic time capsule, the two-foot-long, 55-pound slice of a jadeite-type boulder tells the story of its own creation and offers clues to the dramatic processes involved. This striking specimen, which was recovered from northern Myanmar (formerly Burma), started as a small vein, or fracture, some 12 miles underground in Earth's crust. The vein was wrenched apart by the shifting of two tectonic places when India collided with Southeast Asia over 35 millions years ago. In the aftermath, mineral-rich fluids from the plunging sea floor rose through cracks in Earth's mantle and deposited jadeite rock into the open vein. Over time, as the tectonic plates continued to rub against one another, this vein of jadeite broke and reformed again and again, producing the exquisite emerald green and white marbling we see today.

"In order to look the way it does, this vein had to be broken and healed thousands of times," says George Harlow, Curator in Earth and Planetary Sciences at the Museum. "Imagine breaking a bone and allowing it to heal with a slight kink in it, then doing it again and again until it looks like a pretzel. That is what happened to this boulder." Long prized as an ornamental stone, jade is the common name for two different kinds of rock that have similar properties: nephrite, a silicate mineral rich in calcium and magnesium, and the rarer, more valuable jadeite, a silicate mineral rich in sodium and aluminum. Sharing the display case with the jadeite slab are two intricately carved jade artifacts from China that date from the early 1900s: a nephrite jade vessel for incense and a jadeite jade carving of Guanyin, the Buddhist goddess of mercy. The display itself joins two other recent acquisitions in the Grand Gallery: a spectacular stibnite unearthed by miners in China and the fossilized, iridescent shell of an ammonite, an extinct marine animal.