The Herkimer diamond occurrence is located on Mount Tzuhalem, approximately 6 kilometres east-northeast of Duncan.

The area is underlain by basalts of the Triassic Karmutsen Formation; these are overlain by limestone of the Quatsino Formation. Locally, the basalts are porphyritic (plagioclase and pyroxene phenocrysts) with common pillow structures and amygdules.

Locally, pockets within the amygdaloidal pillow basalts host double terminated quartz crystals, known as Herkimer diamonds.

Herkimer diamonds were first discovered by the Cowichan Valley Rock hound Club in 2006. In 2007 and 2008, the showing was explored by Kevin Root of Beaver Creek Prospecting. Exploration work included reconnaissance geological mapping and rock chip sampling.