In 1973, Chlorastrolite became Michigan’s State Gemstone. A gemstone is a rock or mineral considered to be precious or semiprecious and is usually cut and polished for jewelry.
1. Chlorastrolite is also known as the Isle Royale greenstone because it is ONLY found on the Isle Royale and the Keweenaw Peninsula of Michigan.
2. Chlorastrolite means “green star stone,” which refers to its translucent, fibrous mineral structure that scatters light.
3. The bundling of the fibers gives it a distinctive mosaic or segmented pattern referred to as turtleback; the pattern roughly resembles the looks of a turtle shell.
4. Chlorastrolite is a variety of the mineral pumpellyite and forms in the cavities of basaltic lava from the cooling gas.
5. Its presence in Michigan is a result of the Midcontinent Rift System, a split in the Earth’s crust that began 1.1 billion years ago.
6. The rift system ran from Kansas up to Lake Superior and then back down to the Detroit area, resulting in a major eruption of lava that took place over the course of 20 million years, forming rock deposits up to 15 miles thick.
7. Most of these lava flows are buried under younger rock, but erosion and glaciation have exposed the rock in the Lake Superior region.
8. Large pieces of chroastrolite are very rare and are typically only found as small rounded pebbles after the basalt has been worn away by water.
9. The best places to find chlorastrolite are in the spoil piles from the copper mining on the Keweenaw Peninsula.
10. Chlorastrolite is also found on the beaches of Isle Royale National Park, but please be aware that collecting anything from the park is illegal.
By: Cory Redman, the GRPM’s Science Curator