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

Geyser opal, also known as Spencer opal, is a precious opal variety that is found in Spencer, Idaho, USA. Geyser opal is formed by the hydrothermal deposition of silica-rich fluids from geysers and hot springs. 

Geyser opal is typically a translucent to opaque opal with a play of color that can range from fiery red and orange to cool blue and green. It is also known for its unique fire opal pattern, which is a series of concentric rings of color.

Formation of Geyser Opal

Geyser opal is formed when hot, silica-rich water from geysers and hot springs seeps into underground cavities or geodes. Over time, the silica precipitates out of solution and deposits on the walls of the cavity, forming layers of opal. 

The deposition of silica can occur in a number of ways. In some cases, the silica is deposited as a gel, which later hardens into opal. In other cases, the silica is deposited as a precipitate, which forms layers of opal.

The color of geyser opal is determined by the impurities that are present in the silica-rich water. Iron impurities can cause opal to be red, orange, or yellow. Nickel impurities can cause opal to be green or blue.

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

Characteristics of Geyser Opal

Geyser opal is typically characterized by its botryoidal or cauliflower-like structure, which is caused by the way the opal deposits layer by layer. It is also known for its high water content, which can range from 10 to 40 percent. This high water content makes geyser opal relatively soft and susceptible to scratching, but it also contributes to its play of colors.

Properties of Geyser Opal:

• Hardness: 5.5-6.5 on the Mohs scale
• Specific gravity: 1.9-2.2
• Refractive index: 1.40-1.50
• Transparency: Translucent to opaque
• Color: Colorless, white, yellow, orange, red, blue, green, black
• Play of color: Strong

Geyser Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a matrix material. Such gems are considerably cheaper than solid opals, yet provide the same play of color. Opal doublets are sometimes coated with a thin layer or dome of clear Quartz to make them more resistant to scratches (opal is a relatively soft gem). These are known as triplets.

To make a triplet the opal is ground perfectly flat parallel to the fire layer until the intense fire shows completely across the surface. Then a flat piece of black Basinite or obsidian is epoxied to the flat fire layer.

The opal is again ground flat on the opposite side until the fire layer is exposed and shows its greatest intensity against the black background. At this time the opal should be about the thickness of a hair. A Crystal cap is applied with epoxy and the opal is ground to its final shape. Thus is an Idaho Opal Triplet created.

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

Where You can find Geyser Opal?

Spencer Opal Mine,  Spencer, Idaho The opals are contained in a rhyolite and obsidian flow with abundant gas pockets. Hydrothermal fluids from hot spring activity filled the pockets with common and precious opal. The cost for digging is $15.00 for adults (13 and up) $10.00 for kids (4-12).

Geyser Opal from Spencer Idaho U.S.A Spenser Opal was deposited in volcanic rocks about 4 million years ago by geysers.