Magnetite

Hardness (Mohs scale): 6-6.5. Color: Dark gray to black, often with a metallic sheen. Luster: Metallic to submetallic, dull on tarnished surfaces. Crystal structure: Isometric, often found in octahedral or dodecahedral crystals, but commonly massive or granular. Cleavage: Indistinct. Specific Gravity: 5.17-5.18. It is strongly magnetic.

Magnetite forms in a variety of geological environments. It can crystallize directly from magma in igneous rocks as an accessory mineral. It also forms in metamorphic rocks through the metamorphism of iron-rich sediments or other iron-bearing minerals. Additionally, it can occur in sedimentary environments as detrital grains. Its formation temperature can vary widely depending on the geological context.

Magnetite is a significant iron ore, used in the production of steel. It is also used in various industrial applications, including heavy media separation (due to its high density), as a pigment, and as an aggregate in high-density concrete. Due to its magnetic properties, it's used in certain magnetic recording media and magnetic fluids.

Magnetite is one of the most common iron ores and has been used by humans for thousands of years. Natural magnets, known as lodestones, are composed of magnetite and were used in early compasses. Its magnetic properties are a key distinguishing feature.

In the field, magnetite can be identified by its dark color, metallic to submetallic luster, and most importantly, its strong attraction to a magnet. This is the most reliable field test. It often occurs as disseminated grains in igneous and metamorphic rocks, or as concentrated deposits in banded iron formations or placers. Common locations include various iron ore deposits worldwide, such as in Sweden, the USA (Adirondacks), and the Great Lakes region. When collecting, the magnetic property is paramount for identification.