What Is Spencer Opal, and Where You Can Find It?

Spencer Opal, also known as Geyser Opal, is a rare and precious variety of opal found exclusively near Spencer, Idaho, USA. Renowned for its vibrant hues and unique geological origins, this gemstone forms through hydrothermal processes in silica-rich fluids expelled by geysers and hot springs.

Spencer Opal is characterized by a diverse color palette, including pink common opal, pink precious opal, white, yellow, blue, green, black, and colorless varieties. The pink hues are particularly distinctive, setting it apart from other opal deposits worldwide. The gemstone typically ranges from translucent to opaque and exhibits a strong play of color.

Formation of Spencer Opal

Spencer Opal is formed through the hydrothermal deposition of silica-rich fluids derived from geysers and hot springs. These fluids permeate underground cavities or geodes within rhyolite-obsidian flows, where silica gradually precipitates and accumulates on the cavity walls. This process, occurring over extended periods, results in layered opal deposits. The deposition may manifest as a gel that solidifies into opal or as a precipitate forming successive layers.

Rare light pink precious opal embedded in rhyolite matrix from the Spencer (Stetler) Opal Mine, Idaho.

Characteristics of Spencer Opal

Spencer Opal exhibits a striking range of colors, including pink common opal, pink precious opal, white, yellow, blue, and other varieties.

The opal typically displays a botryoidal or cauliflower-like structure, a result of its layered deposition process. With a water content ranging from 10 to 40 percent, Spencer Opal is relatively soft, registering between 5.5 and 6.5 on the Mohs hardness scale, and is prone to scratching. This high water content, however, is integral to its vibrant play of color, a feature that elevates its value.

What Is Geyser Opal, and Where You Can Find It? Precious opal (Spencer Opal Mine, Spencer, Idaho, USA)

Key Properties:

• Hardness: 5.5–6.5 (Mohs scale)
• Specific Gravity: 1.9–2.2
• Refractive Index: 1.40–1.50
• Transparency: Translucent to opaque
• Color Range: Colorless, white, yellow, orange, red, blue, green, black
• Play of Color: Strong

Precious opal (Spencer Opal Mine, Spencer, Idaho, USA). Photo:James St. John

Spencer Opal Doublets and Triplets in Jewelry

Spencer Opal is frequently crafted into doublets and triplets for jewelry applications due to its inherent brittleness, heat sensitivity, and susceptibility to water loss, which can cause cracking. Doublets consist of a thin slice of precious opal adhered to a supportive matrix, such as black basinite or obsidian, offering a cost-effective alternative to solid opals while retaining the gemstone’s vibrant play of color. This construction enhances the opal’s durability, making it suitable for practical use.

Triplets, an advanced form of doublets, incorporate an additional layer to further improve resilience and appearance. The creation of a triplet involves a meticulous process: the opal is ground flat parallel to its fire layer until the intense play of color is fully exposed across the surface. A flat piece of black basinite or obsidian is then epoxied to this fire layer, serving as a dark backing to amplify the opal’s color intensity. The opposite side of the opal is ground again until the fire layer is revealed, reducing the opal to approximately the thickness of a hair. Finally, a clear quartz cap is applied with epoxy, and the gemstone is shaped into its final form. This quartz layer enhances scratch resistance, given opal’s relative softness, ensuring the triplet’s suitability for everyday wear.

Spencer Opal Mine

Spencer Opal Mine - Idaho

The Spencer Opal Mine, located in Spencer, Idaho—approximately 60 miles northwest of Rexburg—is the principal source of this gemstone. As Idaho’s largest privately owned gemstone producer, it serves as both a commercial operation and a visitor destination. The mine operates daily from 9:00 AM to 6:00 PM, except Tuesdays and Wednesdays, allowing individuals to search for opals. Visitors must bring their own tools, such as hammers and garden claws, and comply with safety requirements, including eye protection and closed-toe shoes.

The opals are embedded in rhyolite and obsidian flows, where gas pockets filled with hydrothermal fluids have deposited both common and precious varieties. Admission for opal hunting is priced at $15.00 for adults (ages 13 and up) and $10.00 for children (ages 4–12). The site also features a gift shop, open until 7:00 PM, a café offering meals, and an online store with varying inventory.

Address: 27 Opal Way, Spencer, ID 83446, United States