Hematite is smooth and non-crystalline. Mostly found in sedimentary rocks, it can also form large replacement deposits. Sources are the United States, Canada, and Brazil. It commonly grows as groups of crystals that form concentric rings, and the bulls eye and ringed patterns in widely varying shades of green make malachite popular among collectors. A decorative stone, it was used for columns and facades of prominent buildings in Russia.
Sources are Zaire, the United States, and Russia.
Instead, chains of silicon and oxygen are packed into tiny sheres. It is this play of color that determines an opal's value. To prevent cracks, opal should be treated periodically with castor oil. The source is Australia. FeS 2 Pyrite is a metallic mineral composed of iron and sulfur.