Land of Nipigon

Amethyst and Agates

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The First 3.5 Billion Years

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How Amethyst and Agates form

Thunderbay area Amethyst and Lake Superior Agates form in very similar ways, they formed during the same time as a result of the same geological event. 

Its all just quartz, plain old white to clear quartz. This has the same chemical formula as glass if you are having problems picturing it. And like glass, if you add specific elements or changes in the conditions around the glass, it will change the colour and properties of the glass.

 

Agates

Agates start their life as a void or hole in a rock deep in the earth. Silica rich fluids (the stuff quartz is made of) work their way through the intense pressures and heat. Along the way they pick up impurities from the surrounding rocks as things are constantly being dissolved into the fluids. 

When a void, or hole in the rock is encountered, a change occurs and some of the fluid hardens quickly. This created a thin layer around the void and is the first “band” of an agate. Over time this happens again and again and the bands build up and fill in the void. The colors of the banding is dependent on the impurities within the fluids forming the agate. They change over time, giving a nice multicolored banding in the agate. 

Agates are made of a very strong rock, quartz. As the voids and holes in the rock are now filled with a material stronger than the rock itself, as the rock erodes away the “hole” resists weathering as the rock erodes around it. These little bubble of quartz then roll around on beaches, break apart eventually and the waves will actually polish them.

 

Looking for agates

Agates are relatively common in area gravel pits as well as sand/rock beaches. The glaciers however did a great job of bulldozing alot of them south into Michigan and west between Duluth and Grand Marais. A keen eye can still spot the little gems throughout our region.

Amethyst

Amethyst is the similar as the fluids work through larger cracks and voids, the liquids cool slower, over time and the crystals get a chance to grow larger. The impurities in the rock are what gives it its colour. In the case of amethyst, the fluids picked up iron in the surrounding rocks and due to natural processes gave the quartz a purple hue.  Thunderbay Amethyst is known for having iron “specks” called inclusions in the purple quartz and in some instances an iron rich coating is present making the amethyst almost red in colour.

The longer the fluid takes to cool, the larger the crystals will get. This is how you get large crystals lining a large cavity – or a geode.

Amethyst mines

By far the easiest way to get some good quality Thunderbay Amethyst is to go to one of the many open amethyst mines which allow hand picking of material. They blast, pile and allow you to dig through the piles to find your own pieces of amethyst. You can quickly fill a bucket so be prepared.

Ana agate starts as a hole in the rock, then it fills in layer by layer.

The fluids will solidify tracing whatever shape is underneath – in  this case the triangle pattern from the amethyst crystals.

Amethyst veins – same as quartz but purpler. Note the red iron rich rock surrounding the veins.

Red hematite inclusions in Thunderbay Amethyst

So much hematite, near red Thunderbay Amethyst

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