Chlorastrolite: Michigan's Striking State Gemstone

Chlorastrolite: Michigan's Striking State Gemstone. Photo: Adam Johnson‎

Isle Royale has long been famous as the home of chlorastrolite, known informally in rock-collecting and lapidary circles as "Isle Royale greenstone."  Cholorastrolite is a variety of pumpellyite:  Ca₂(Mg,Fe)Al₂(SiO₄)(Si₂O₇)(OH)₂·H₂O.

In 1972 the governor of Michigan signed a bill designating chlorastrolite as the official state gem. We often hear, "I thought Petoskey Stone was the State Stone?!" The answer is, Petoskey Stone is the State Stone, and Greenstone is the State GEMstone."

Geology

Formed after hot lava flows settled and hardened about 1 billion years ago. Chlorastrolite, meaning "green star stone," occurs as amygdules or cavity fillings in certain of the lava flows on Isle Royale. When the water and wave action has worn away the basalt, they are found as beach pebbles and granules in loose sediments., generally greenish in color. When polished, either by wave action on the beaches or artificially, the "greenstones" generally exhibit a distinctive and attractive mosaic or segmented pattern, sometimes referred to as "turtleback".

Isle Royale Greenstone, the mineral, can be found in a belt of greenstone, the rock. Greenstone Ridge, one of the earth’s largest and thickest lava flows, is up to 800 feet thick and extends deep under Lake Superior in a continuous flow that reappears 50 miles later on the Keweenaw Peninsula in Michigan.

In Isle Royale National Park, the gemstone that bears its name can be found encased in the basalt (hardened volcanic rock) on the island, or if weathered out of the basalt, can be found as pea-sized pebbles on the shore.

The polished stones also commonly are chatoyant - the property of having a luster resembling the changing luster of the eye of a cat. Chatoyancy is probably best known in the gemstone called tiger eye and is a property of translucent material that contains fibrous structures capable of' scattering light. The grouping together of bundles of such fibers produces the mosaic pattern of the "greenstones."

Chlorastrolite (Isle Royale Greenstone).
Photo: Piscator629/reddit

Greenstones have a hardness of 5-6 and are very well suited for all varieties of jewelry.  Polishing is tricky, as some stones have quartz or calcite centers, are hollow, and the most important consideration is the rapidly changing cell and color pattern as a stone is sanded to shape.

Soft chlorite inclusions on some of the mainland gemstones are a problem which can ruin a great pattern.  The toughness of this stone allows for cutting and polishing of all types of cabachon shapes, calibrated or freeform, and is a great material for inlays.

It is found in the Keweenaw Peninsula of the Upper Peninsula of Michigan and Isle Royale in Lake Superior. Isle Royale is a National Park, and so it is illegal to collect specimens there. so the Keweenaw Peninsula is your best bet for hunting down this Michigan treasure.

Chlorastrolite. Photo: Jeff Kopsi

See also: 
Benitoite: The Rarest Gemstone Found Only in California
Top Spots For Gem Hunting In The US II
Larimar: The Blue Stones of Atlantis