mineral

Magnetite / Iron-Rich Concretion

Magnetite (Fe3O4) or Bog Iron Limonite

Hardness: 5.5-6.5 on Mohs scale; Color: Black, brownish-black, to dark grey; Luster: Metallic to sub-metallic/earthy; Crystal Structure: Isometric (often octahedral, though this specimen is rounded/weathered); Specific Gravity: 5.1-5.2.

Hardness: 5
Color: Black, brownish-black, to dark grey
Luster: Metallic to sub-metallic/earthy

Identified March 29, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the cooling of magma in igneous rocks or through chemical precipitation in sedimentary environments (like bog iron). Often found as water-worn pebbles in stream beds or glacial till. Age can vary from billions of years (Precambrian) to relatively recent Holocene deposits.

Uses & applications

Primary ore of iron; used in heavy media separation, as a pigment (mars black), and in magnets. Small water-worn stones like this are often kept as pocket stones or curios for their magnetic properties.

Geological facts

Magnetite is the most magnetic of all the naturally-occurring minerals on Earth. Naturally magnetized pieces of magnetite, called lodestones, were used as the first magnetic compasses in history.

Field identification & locations

Identify in the field using a strong neodymium magnet to test for attraction. It will leave a black streak if rubbed on unglazed porcelain. Commonly found in 'black sands' on beaches or in iron-rich geological regions like the Lake Superior area or the Adirondacks.

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