Co-mingled iron oxides
.........something akin to meteorites have been found embedded in the plants and the circles at some sites.
>"In 1993 a crop formation at Cherhill, England was discovered in which some of the epicenter plants were coated with an iron "glaze," composed of fused particles of apparent meteoritic origin. This coating consisted of comingled iron oxides (hematite and magnetite) fused into a heterogeneous mass, which was actually embedded in some of the plant tissue."
This is the experiment that the MIT students were trying to reproduce.
You can read the short report <here>