Know Your Rocks

Prehnite

Prehnite is a hydrothermal mineral commonly found associated with Copper and Silver in the basalts of the Keweenaw Peninsula. We now find Prehnite filling veins and vesicles in these basalts in spoil piles of the mine dumps in the Keweenaw Peninsula. Prehnite can also be found eroded out of basalt in the hills and are popular beach pebbles.

If Prehnite forms in veins, as it does in the Keweenaw Peninsula, it generally associates itself with native copper. I love to find these pieces, with Copper inclusions. These copper-included pieces make lovely jewelry, with the apple green of the Prehnite surrounding the shiny copper.

In this vein Prehnite, one cannot notice the chatoyant needle-like crystal structure normally associated with larger non-vein Prehnite pieces such as the gemmy pieces found in Australia and other localities around the world. Feather-like Prehnite crystals are seen in U. P. Prehnite. This is actually variegated and non-variegated Prehnite.   Pink, red, and green, and other color combinations can be found, depending on what minerals flowed into the vesicles and veins along with the Prehnite. Locals refer to pastel Prehnite permeated with copper flecks found on the beaches in the Calumet, and also in the Copper Harbor area as “Patricianite”. These Prehnite pebbles can be jewelry grade, and are wonderful to collect as they roll in with the Lake Superior breakers.

For many years it was thought eyed Prehnite was Thomsonite. After being analyzed by Michigan Tech, the truth was discovered. Locals still call this gorgeous chatoyant-eyed Prehnite U.P. Thomsonite.

Wonderful stone jewelry is made from U.P. Thomsonite. Prehnite in the Keweenaw can vary in color from pale white to apple green, to pink. Pink Prehnite is colored by microscopic copper inclusions.