Limonite

Limonite – FeO(OH)·nH2O
Elements: Iron, Oxygen, Hydrogen
Class: Oxides – Hydroxide
Crystal System: Amorphous
Hardness: 4-5.5
Apatite
Talc (1)Diamond (10)
Softer than glass. A steel knife scratches it.
Origin: Qinglong Mine, Qianxinan, Guizhou – China
Did you know?

Limonite is not even a true species, just a mix of rusty iron oxides, yet it is a master forger. Replacing pyrite atom by atom, it can keep the original cube shapes intact, leaving rust-colored boxes collectors call devil's dice.