Quartz

specimen number:
2292275

location:
Dryhead Agate Mine, Big Horn Co., Montana, USA

description:
A cut and polished slab of what is known as dry head agate.

Dryhead agates are highly prized fortification agates found in the Dryhead region of south-central Montana, situated between the Big Horn Mountains and the Pryor Mountains. Geologically, these agates formed within the limestone marine strata of the Mississippian-age Madison Group, where silica-rich hydrothermal fluids filled prehistoric dissolution cavities and vugs over millions of years, depositing tight, concentric bands of chalcedony with rich tones of orange, red, brown, and white. Commercial mining and surface quarrying of the primary deposit on the Dryhead Ranch began in the early 1950s, driven by a growing post-war lapidary hobbyist market. The mining operation was worked intermittently under private leases and family-run concessions, reaching its peak activity during the 1960s and 1970s. While large-scale commercial extraction has since ceased, the locality continues to produce small quantities of fine material through sporadic, limited diggings on private land into the 2020s.

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