This past fall I paid a visit to my grandparents in New Berlin, IL for their 50th wedding anniversity (congrats Grammy and Gruncle!) and couldn’t miss the chance to visit the Illinois/Iowa border, an area renowned for its geodes. I collected geodes in a creek near Perry, IL during a previous visit but only found calcite geodes with no void space, which are crazy tough to open and aren’t especially pretty since you can’t see natural crystal faces, only cleavage planes and fractures that resulted from smashing them with a sledge hammer.
However, the geodes I found in the creeks of Keokuk, IA and Hamilton, IL (Keokuk’s cross-Mississippi River neighbor) are spectacular. I couldn’t tell you the names of the creeks I searched in, however. I just drove around until I found creeks with easy foot access. This worked well most of the time, but I would recommend checking to see if the creeks are on private property or risk getting kicked out for trespassing like I did (Illinois folks can be ornery at times).
The majority of the geodes come from the Warsaw Formation and are found both weathered out of the rock beds as well as still in place in the creek banks. Some locations contain vugs as well that exhibit the same mineralogies as the geodes.
There’s no easy way to open these geodes. Some people use chain pipe cutters to open larger geodes, but these tools are expensive. The simplest way to open them is to hit them with a hammer until they break, however you risk damaging some of the more delicate minerals (although it’s way more fun than I’m willing to admit). I use a slightly different method with a chisel. I chisel a line following the entire circumference of the geode, forming a plane of weakness or artificial fracture plane. Then I hammer it along the line until it breaks open, usually resulting in a relatively clean break. I do this to ensure that I don’t damage the dolomite crystals that are usually only surficially attached to the primary minerals.
One of the great mysteries of the geode is how it’s formed. One of the most compelling hypotheses is a void space created by some means, for example the decomposition of an organism trapped in a layer of sediment, that is filled in with mineral precipitates. Most siliceous geodes (and some carbonate geodes) contain a layer of chalcedony, or cryptocrystalline silica, lining the outside wall. Some geodes only contain chalcedony.
But more commonly, the primary mineral is either quartz (amethyst, smoky quartz, colorless, etc.) or calcite. The easiest way to distinguish between these two is by scratching the mineral in question with a penny. A penny will scratch calcite but not quartz. You can also look at the crystal habit to distinguish between them. Calcite crystals with natural crystal faces (and cleavages) will be rhombohedral in shape, whereas quartz will be hexagonal and occasionally pyramidal. Other minerals I’ve identified in the geodes are dolomite (a calcium/magnesium carbonate), sphalerite (zinc sulfide), and kaolinite (an aluminum-bearing clay mineral).
If weather permits, I’ll make an exciting trip to Rhea County, TN this weekend (for what will remain a secret). If you have any requests for how-to videos or nature trips, feel free to email me at email@example.com.
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