Gneiss, amphibolite, schist, skarn, and igneous rocks, as well as alluvium formed from the weathering and erosion of these rocks, are the primary sources of Industrial Garnet. There are garnet mines and occurrences in 21 States, but only three are currently operating (2006): one in southeastern Montana, one in northern Idaho, and one in eastern New York (unweathered bedrock; two mines).
The Wallace Formation of the Proterozoic Belt Supergroup, which is associated with garnetiferous metapelites, is where Industrial Garnet is mined in Idaho. Tertiary and (or) Quaternary sedimentary deposits are nearby. The crystalline rocks of the Adirondack Mountains, which are a part of the Proterozoic Grenville province, and the southern Taconic Range, which is a part of the northern Appalachian Mountains, are mined for garnet in New York. Amphibolite, mica schist, gneiss of Archean age, and younger granite are among the sources of garnet in placers in Montana. Garnet was made from saprolite and gold dredge tailings at two recently closed mines in southwest Montana.
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