Our brains like round spaces. In archetecture, this falls under the topic of Topophilia, where thoughtful space planning results in a more comfortable, and some would say inspirational, environment. From a spiritual perspective, the orthagonal represents the earthbound and circles the realm of the heavens. From kivas, to yurts, to round churches that keep the devil from hiding in corners, humans have always created round living spaces for both the spiritual and the mundane comfort. (I've another hypothesis concerning kivas, but I'm saving that for a blog post, one of these days.) Myself, I don't subscribe to powerful forces at work, just the simple fact that our brains find it comfortable. Here's an interesting example I came across lately (I'm still trying to find the link): Have you ever noticed that the latest versions of Windows rounds the edges on the window panes? It turns out that it is easier to mentally process a circle than a square and that by just rounding the corners a little bit, the square becomes much easier to metally process. Edges and corners demand our attention, smooth shapes take less thought, and are therefore relaxing. I've a friend who lives in a house that was built inspired by the rounded shapes of barnacles, although everyone refers to it as the "Mushroom house". It has a 10 second cameo in the movie Sleeper. The inside of the house is just wonderful: open, bright, and relaxing to be in. An archetect friend of mine was challenged by a client who did not like the rounded wall he had designed, insisting that humans want sharp corners and orthagonal shapes. My friend said that he'd be happy to redo the design to the client's wishes, and would even get right to work designing a square toilet seat to be included in the house.

Here's a blog post with more thoughts on this, and another from the same blogger, and here's a teaser from a recent issue of Scientific American.

Concerning the magnetite fused to the grain stalks: certainly you have played with sparklers, which use small metal particles to create the incandescent sparks. Ever notice the tiny little balls that fall to the pavement? It would not be that big a deal to wave a sparkler around the newly created circle (especially if the stalks were damp with morning dew) and introduce a strange wrinkle for you investigators to puzzle over. How many accounts include "weird lights"? I recommend you try this in a controlled experiment and send the sample to other investigators for comparison. (PS: most sparklers contain aluminum powder, but you can make your own using iron particles)