Chlorastrolite Value Guide

Chlorastrolite, famously known as 'Isle Royale Greenstone', is a rare Precambrian mineral found within the basaltic lava flows of Michigan's Keweenaw Peninsula. It is highly collectible due to its unique stellate growth patterns and its status as the official state gem of Michigan since 1971.

What Is Chlorastrolite Worth?

The typical price range for Chlorastrolite is $25 - $35 based on recent sales. However, values can vary depending on the item's condition, rarity, and other factors such as:

• The clarity and definition of the 'turtleback' pattern significantly impact the market price.
• Size and weight of the specimen, with larger, high-quality nodules being exceptionally rare.
• The quality of the polish and the intensity of the chatoyancy visible on the stone's surface.
• Authenticity of the find location, as collecting is now strictly prohibited within Isle Royale National Park.

How to Identify Chlorastrolite?

• Look for the distinctive 'turtleback' or stellate pattern consisting of radiating green and black fibrous crystals.
• Observe the chatoyant effect, where light reflects off the fibrous structure to create a shimmering, cat's-eye appearance.
• Verify the origin as the Lake Superior region, specifically Isle Royale or the Keweenaw Peninsula, where it formed in amygdaloidal cavities.
• Check for a hardness of 5 to 6 on the Mohs scale, typical for this variety of pumpellyite.

History of Chlorastrolite

Chlorastrolite is a rare mineral found primarily in the Keweenaw Peninsula of Michigan, particularly on Isle Royale and the surrounding Great Lakes region. It formed within amygdaloidal cavities of basaltic lava flows. Its discovery led to its designation as Michigan's official state gem in 1971, cherished for its unique 'turtleback' or 'stellate' patterns when polished.