Isle Royale Greenstone
Isle Royale Greenstone
Isle Royale Greenstones are an enduring symbol of Michigan's significant cultural and mining heritage. Today, the majority of gem-quality greenstones utilized in jewelry originate from the reclaimed tailings of historic mines on Michigan's Keweenaw Peninsula. The quest for these elusive and uncommon gems has consistently presented a challenge. We offer these stunning greenstones as a testament to our excellent craftsmanship and care.
Isle Royale Greenstone
Part 1 Excellence in Greenstones - Series - This series we will discuss characteristics in greenstones that lend to making better jewelry with the Isle Royale Greenstone. These characteristics are sometimes because of the composition of the greenstone, but also the decisions made by the cutter that is making the gem. It's time to explore together the ways we can make better greenstones and use them in jewelry.
What is Isle Royale Greenstone?
Isle Royale greenstone, also known as chlorastrolite (its mineral name), has several other common names, including:
Greenstone: This is a widely used and simpler name, often used interchangeably with Isle Royale greenstone.
Michigan Greenstone: This name emphasizes its primary location of origin in Michigan.
Green Starstone: This name comes from the Greek words "chloros" (green), "aster" (star), and "lithos" (stone), referring to its green color and the radiating, star-like patterns within the stone that create a chatoyant effect.
Turtleback: This nickname describes the distinctive mosaic or segmented pattern seen in polished stones, which resembles the markings on a turtle's shell.
Lake Superior Greenstone: This name highlights its association with the Lake Superior region.
Isle Royale greenstone, scientifically known as chlorastrolite, is a unique and rare gemstone primarily found in the Lake Superior region of North America, particularly on Isle Royale and the Keweenaw Peninsula of Michigan. It is the official state gem of Michigan.
Here's a breakdown of its definition and key characteristics:
- Mineral Composition: Chlorastrolite is a variety of the mineral pumpellyite, a hydrous calcium aluminum silicate hydroxide with magnesium and iron.
- Color: It typically exhibits a range of green hues, from light olive to deep bluish-green and even greenish-black. Some specimens can have brown, white, blue, or pinkish-brown areas due to inclusions of other minerals.
- Appearance: A defining feature of Isle Royale greenstone is its distinctive "turtleback" pattern. This mosaic or segmented appearance is created by finely radiating or stellate (star-like) masses of fibrous crystals.
- Chatoyancy: These radiating fibers often display chatoyancy, a shimmering or cat's-eye effect where a band of light appears to move across the surface as the viewing angle changes. This gives the stone a varying luster, from subtranslucent to opaque. The name "chlorastrolite" itself comes from the Greek words "chloros" (green) and "astron" (star), referring to this characteristic.
- Formation: Isle Royale greenstone formed about 1.1 billion years ago within the gas cavities (vesicles) of ancient basaltic lava flows associated with the Midcontinent Rift System. Over time, hot, mineral-rich fluids filled these cavities, depositing the chlorastrolite.
- Occurrence: While named after Isle Royale, collecting greenstone is now prohibited in the national park. The gem can still be found in the Keweenaw Peninsula, often in the spoil piles of old copper mines.
- Rarity: Gem-quality chlorastrolite is considered quite rare, and larger pieces are especially uncommon.
- Uses: It is primarily used in jewelry, often cut into cabochons to showcase its color and unique patterns.
In essence, Isle Royale greenstone (chlorastrolite) is a beautiful and rare green gemstone, prized for its distinctive "turtleback" pattern and chatoyancy, found primarily in Michigan's Upper Peninsula and formed within ancient lava flows.
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