History Gem – Michigan’s Glacial Gemstones of Northeastern Upper Peninsula May 2020

In 1972 the founder of the Gitche Gumee Museum, Axel Niemi, published the first of what would be three printings of his book. Although the publication covers rocks and minerals other than agate, the main focus of the book is the Lake Superior agate. The book is around 32 pages long and is 5” wide and 7.5” tall. The first chapter is included below (slightly edited).

KNOW YOUR AGATES

How to tell agates from other beach stones.

If you should find a smooth, brown beach stone, chances are that you have a common chert, light brown smooth, and waxy on the surface. An agate has similar qualities, except for a few differences.

1. Agate is translucent, which means light passes through it. Another stone, jasper-agate is opaque, although otherwise similar in markings to agate. No light passes through jasper.
2. Agate is colorful and can be red, red-brown, whitish, brown, sometimes green, black, or blue. Agates can also be combinations of these colors.
3. Common chert found on Lake Superior beaches is opaque. Colors can be grey, tan, or brown. Like jasper, chert is opaque.
4. Picture or scenic chert is tan and purple-lavender, making a pretty cabochon. A cabochon is an oval or cushion-shaped dome fashioned by the lapidary grinder to make the stone into a form used in jewelry.
5. Agate may be found as a whole nodule, or as a part of or chip from a whole agate. In order to understand agate better, we must necessarily know how agate is formed. We must go back millions of years to the period when the Killarney Mountains were formed (now known as the Midcontinental Rift). The lava cooled leaving empty seams and bubble-pockets that hardened. Since the volcanic rock is porous, mineral-filled liquids seeped into these pockets to slowly fill them. The liquid was probably water in the form of superheated steam from below or rainwater from above. Some authorities believe the mineral-rich water simply seeped into the pockets leaving the silicon dioxide and coloring agents behind to fill in the pockets. Another theory has it that silicone gel-filled the pockets and then slowly crystallized into the agate with various beautiful patterns. After millions of years of erosion, glacial wear and transportation, weathering, frost action, and other natural forces — a fracture-free Lake Superior agate is quite rare. Most agates are of the reddish or reddish-brown color due to the high content of iron in this area (iron oxide in solution was deposited together with the silica).

All agates have a cool, waxy, rounded look when beach-worn. Chipped pieces have a smooth or rippled surface appearance. Chert has a similar appearance since like agate, it fractures in a conchoidal manner.

Old rockhounds never die—they just petrify!