Spencer Opal

Spencer Opal comes from the Spencer Opal Mine near Spencer in eastern Idaho, USA, a popular fee-dig locality. The opal occurs as thin, brightly colored seams deposited in layers within volcanic rock.

Because the precious color layers are so thin, Spencer opal is most often fashioned into triplets, a slice of opal sandwiched between a dark backing and a clear quartz or glass cap, to produce durable, vivid gems. Solid pieces are less common and prized.

The mine is well known to rockhounds and tourists, who can dig their own material, making Spencer a familiar name in American opal collecting.

Spencer Opal formed in volcanic rock when silica-rich, low-temperature waters filled thin cavities and fractures, depositing hydrated silica in fine layers. The opal precipitated between successive flows or in gas pockets of the volcanic sequence.

The play-of-color arises from orderly stacks of uniform silica spheres in the thin opal seams. Because deposition occurred in narrow openings, the precious color layers are typically only millimeters thick.

This layered, thin-seam volcanic occurrence is what makes triplet cutting the practical way to use much Spencer material.

Identify Spencer opal by bright play-of-color in very thin layers, frequently presented as triplets. Viewed from the side, a triplet shows three distinct layers: dark base, thin opal, and clear cap.

The opal itself has the usual properties: hardness about 5.5-6.5, white streak, conchoidal fracture. The clear cap of a triplet may be glass or quartz and feels harder than opal.

Look-alikes include other thin-seam opals and assembled stones. The key clue is the triplet construction (visible layering from the side and a flat, glassy top), which distinguishes assembled Spencer stones from solid natural opal. Solid Spencer opal is rarer and shows color throughout its depth.

Spencer Opal is used in jewelry, predominantly as triplets that turn its thin color seams into durable, attractive cabochons for pendants, rings, and earrings. Triplets are also more affordable than solid opal.

The Spencer Opal Mine is a noted fee-dig destination, so the material has recreational and souvenir value for rockhounds. Triplets are reasonably durable but the bonded layers should be kept out of prolonged water and ultrasonic cleaners, which can damage the adhesive.

Solid Spencer opal pieces, being scarcer, are sought by collectors who prefer natural, unassembled stones.