Devils dice limonite after pyrite 1-2cm

This naturally shaped Limonite Iron Crystal formed from the process of psudomorphing Limonite after Pyrite. This process involves one mineral being substituted for another while retaining the original minerals form.
Limonite is an ore consisting in a mixture of hydrated iron(III) oxide-hydroxide of varying composition. The generic formula is frequently written as FeO(OH)nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.

Together with hematite, it has been mined as ore for the production of iron. Limonite is heavy and yellowish-brown. It is a very common amorphous substance though can be tricky to find when mined with hematite and bog ore.

Limonite has been known to form pseudomorphs after other minerals such as pyrite, meaning that the chemical weathering transforms the crystal of pyrite into limonite but keeps the external shape of the pyrite crystal. It has also been formed from other iron oxides, hematite and magnetite; the carbonate siderite and iron rich silicates like some garnets.