Epidote

Epidote

The Rock Identifier app says this is Epidote. It matches images I found online (the stone, not the crystal). Different AI sources also agree. The Mohs scratch test for hardness scale points to Epidote. A very experienced rock expert agrees. Here is what Rock Identifier says about Epidote:

Epidote isn’t one of the most common minerals to stumble across, but when you do, hold on with both hands! This is an incredibly precious mineral for collectors. Although it isn’t classified as a precious gemstone, the relative rarity makes it a genuine treasure. They are most commonly seen among ancient lava fields and metamorphosed rock.

Formation of Epidote

Rock Identifier:

Its presence is noted in metamorphic rocks, such as marble and schist, where intense heat and pressure have altered pre-existing minerals. Furthermore, it arises from hydrothermal alteration, a process where hot, chemically active fluids interact with minerals like feldspars, micas, and garnets within igneous rocks, leading to the creation of Epidote.

I found this specimen while rockhounding along the banks of a fluvial river in northwest, Wisconsin. This includes the Midcontinent Rift area, which matches the geological requirements for the formation of Epidote. The stone is not tumbled or polished. It is rubbed with oil to saturate the colors to aid in identification.

Midcontinent Rift – Very Simplified

To simplify things for this post, the rift area is a very large zone. Tectonic activity tried to split our continent in half. Volcanic activity filled the rift valley with lava flows. It also infused the area with igneous rocks and metamorphosed rocks. Additionally, it deposited many valuable minerals. The glaciers effectively covered the evidence of the rift in most places.

Map of the Midcontinent Rift System