Lake Superior Agates, Gem Stones & Rocks - Part 1
Igneous and Metamorphic Lake Superior Beach Rocks

Basalt

Basalt is one of the most common rocks you'll find, yet I love their smooth surface and solid feel in my hand. Each one has a distinctive shape and in a variety of shades of color from blush-black to grey. They are volcanic rock formed from lava that quickly cooled when it reached the surface. That quick cooling is what caused it to be dense, very fine grained (tiny crystals) and smooth - although the smooth surface is also due to glacial grinding & Gitche Gumme tossing it about for eons. That Basalt rock you hold in your hand is over a billion years old!

Ophitic Basalt

Ophitic Basalt looks like a basalt rock that has been decorated with lighter colored little painted petals. They come from tiny feldspar crystals that were in the lava. Because the crystals have worn at different rate than the basalt there is often a slight mottled texture to these these stones. This particular specimen has many feldspar crystals, others have fewer and require more close inpsection to see them (look for the little spots that look as if they were put there by dabbing with a paint brush).

Rhyolite

Rhyolite is a sandy-colored version of Basalt. Formed from quickly cooling lava just like basalt, it is rich in silica and potassium, whereas basalt is poor in these minerals and richer in iron, and other minerals. Coloration of Rhyolite will vary depending upon the mix of silica and iron and some of the trace minerals. Some stones may be a difficult call: basalt or rhyolite?

The sample on the far right may be Rhyolite that has darker bands of flow of an iron-richer form of lava. > > >

Vesicular Rhyolite

Frequently the molten lava flowing from deep down in the earth was filled with gases that formed bubbles when the lava cooled - leaving a pock-marked surface on the rock. The sample to the right shows two examples - one with many vesicules and the other with only a few.

A similar process occurs with basalt, forming vesicular basalt. Unfortunately I don't have any samples to show here.

Amygdaloidal Rhyolite

Sometimes the holes that create vesicular rhyolite fill up with molten minerals - often calcite and quartz - that form crystals in the cavities when the lava cools into a rock. The crystal spots are always rounded since the holes were formed by gas bubbles.

Here is a sample of Amygdaloidal Basalt > > >

Porphitic Basalt

At first glance, a porphitic rock looks like the amygdaloidal rocks pictured above. On closer inspection you'll see the blobs of crystals are rectangular shaped not round. This is because the crystals were already there when the magma was molten. Rhyolite can also occur in a porphitic form.

Granite

Granite is formed deep underground and stays there for a time, cooling slowly. This process forms large crystals and includes minerals like quartz, feldspar and mica embedded within the rock. It is coarse grained and speckled. Although it appears quite different, it is similar in basic composition to rhyolite - rich in silica and poor in iron.

Gabbro

As granite is the coarse grained, slow cooling cousin of rhyolite, gabbro is the coarse cousin of basalt, formed deep underground as granite. My guide books say it can range from black to gray or a mixture of black and light crystals. Sparky says to look for "weathered white flecked" dark, coarse rocks. The white or light gray crystals you see in the sample to the right are weathered plagioclase crystals.

Diabase

Here's a rock that is a hybrid of basalt and gabbro - diabase. Basalt results from lava that cools quickly on the surface; gabbro results from magma cooling slowly deep within the earth; diabase occurs in between, cooling medium slowly and closer to the surface. As a result it is medium grained with smaller crystals than either granite or gabbro.

If you hold these guys at an angle to the sun, you can usually see some smaller sparkly crystals reflecting the light.

Slate

This is a metamorphic rock formed by heat and presure on shale. It is dark to gray in color, smooth, somewhat shiny, and generally flat. Look for some layering. Look closely at this sample and you can see the lighter colored layer sandwich.

Gneiss

Gneiss is also a metamorphic rock. It can be similar in appearance to granite but can be more fine-grained. What is distinctive about gneiss are the bands of minerals that form alternating colors.

Agates

Part 2. More pictures and descriptions of other types of rocks found on Lake Superior beaches plus a few pix of Lake Superior Agates.

Part 3. Identifying Agates vs. Agate Wanna-Bees.

Want to See More Agates - Go Here for pix from the Lake Superior Agate Museum.

See more Lake Superior Agate Pictures plus some advice on using tumbling pellets to tumble and polish agates.

Black River Harbor Agate Hunting - pictures and video.