Talk:Magnetite/@comment-26507296-20150702072033/@comment-25145798-20150702073111

The formation of magnetite during the geological metamorphism of limestone only occurs with limestone that formed before oxygen levels on Earth were sufficient for dissolved iron to accumulate in the oceans. This iron reached levels where it clumped and solidified out of the ocean water, forming iron ores on the sea floor. Some of this iron ore, when undergoing geologic processes involving heat and pressure, became magnetite. Photosynthetic organisms have since greatly increased the levels of oxygen on Earth, and iron no longer persists in the oceans, as it is oxidized too quickly. (Source ) So, in that case, the mushrooms are high in iron minerals, and have gone through some form of magma stress or metamorphism.