Popular Rocks and Minerals of Wisconsin

Granite

Granite is the most common igneous rock in Earth’s continental crust. Granite can be found all around you in countertops, tiles, monuments, and more.

Granite is made up primarily of feldspar, quartz, and mica, with feldspar being the most prominent. Feldspar is usually white gray, pink, or red in color and features a visible cleavage pattern. The mica found in granite can include muscovite, biotite, or both.

Granite is formed by slowly cooling molten rock material. Because the process is so slow, the crystalline structures of the minerals that make up granite are able to grow quite large. Over time, granite is uplifted and revealed on the Earth’s surface via the erosion of overlying sediment.

Granite 23 by James St. John. This file is licensed under the Creative Commons Attribution 2.0 Generic license.

Galena

Streak: Lead-gray

Hardness: 2.5

Chemical Formula: PbS

Galena is the natural mineral form of Lead Sulfide, the most important ore of lead and silver. Galena is one of the most abundant sulfide minerals, a group which contains other popular minerals such as pyrite and cinnabar.

When fresh or tested on a streak plate, galena is opaque and a bright metallic gray. Galena can be found as a cubic, octahedral, or dodecahedral crystalline structure or a combination of these.

Galena deposits are found worldwide with notable deposits in the Lead Belt in the United States, the Sullivan Mine of British Columbia, and in the Driftless Area of Illinois, Iowa, and Wisconsin.

There is historically evidence to support the use of galena as far back as 6500 BCE as lead beads. Galena was also used in Ancient Egypt in cosmetic eye kohl.

Copper

Streak: Rose

Hardness: 2.5 – 3

Chemical Formula: Cu

Copper is one of the few metals with a natural color other than gray or silver. When it is pure, copper is a rose color, but it quickly tarnishes and later oxidizes after time in the air. This tarnish creates the green color the copper is known for.

Copper crystals are incredibly rare and copper is most often found in irregular or branching masses.

Copper is thought to be the oldest metal in use, with examples of early use being found as early as 8000BCE. By 3500BCE, copper was being mixed with tin to create bronze, ushering in the Bronze Age.