Lake Superior shoreline

Agates are a variegated form of chalcedony with crystallized quartz formed at the microscopic level. They are formed from lava flows in which, as it settles, gaseous pockets develop. Molten basalt is typically the substrate for the activity. Over time, silica-rich fluids enter these pockets and form micro-crystals coating the sides of the pockets. Different rates of flow and various minerals contribute to a large variety of agates in a wide range of colours. These nodules are typically harder than the substrate and, over time, are exposed as the host material wears away. The agate’s surface can be pitted and scored, retaining the inner contours of the original cavity. Agates can also form within sedimentary rock, including dolomite, sandstone, and limestone, in which organic materials have been cast. As they break down, the silicate materials fill the void. 

Similar to agate is jasper, a crypto-crystalline quartz formed at relatively low temperatures (around 200° C). It is formed when silica-rich water filters in through the cracks and pockets of cooling volcanic material. It is found in a variety of colours, although red and green are the most commonly identified.

Why does this region hold such a rich quantity of these treasures? The Great Lakes formed over a billion years ago, when North America began to shift and split, creating the Midcontinent Rift System, and the Canadian Shield was a vast, mountainous lava field. The Superior trough eventually became the basin for the largest of the Lakes. As the basaltic lava cooled, the trapped gases formed vesicles in the rock, initiating the process of agatization. 

In more recent times (within the last 100,000 years), a massive glacier known as the Laurentide Ice Sheet covered the entire region. The tremendous pressure of the mile-thick ice ground the surface of the Shield, exposing the hardened agates and depositing them along the region as the glacier receded. Rivers washed the stones down to the shores of the Lakes, particularly Superior. 

Viewing stones of agate material are especially noticeable as their glassy textures give them a highly prized patina. Although most of the samples found are naturally tumbled by the Lake’s wave action, occasionally collectors can find one suitable for use as a viewing stone.