Fire Opal: Properties, Formation, Occurrence

Fire opal is a variety of opal that characterized by its vibrant orange, red, and yellow hues, resembling fiery flames.

Fire opals are prized for their captivating play of color, a phenomenon that occurs when light interacts with the opal's silica spheres. As the opal is tilted, different colors emerge, creating a mesmerizing dance of light. The intensity and variety of the play-of-color depend on the opal's structure and the size of its silica spheres.

Formation of Fire Opal

Fire opal is formed in a similar way to other opals, through the precipitation of silica gel from hydrothermal fluids. However, The body color of fire opal is caused by the presence of iron oxide impurities in the silica gel that forms the opal. The play-of-color is caused by the diffraction of light as it passes through the tiny spheres of silica that make up the opal. Fire opal is typically found in volcanic rocks, such as rhyolite and basalt. It is also found in sedimentary rocks that have been altered by hydrothermal fluids.

Stunning Mexican Fire Opal. Photo: Jeff Schultz

Characteristics of Fire Opal

Fire opal glows with the fire of the sun: hot honey yellows, oranges, and reds so bright they look as though they might glow in the dark. Fire opal sometimes only occasionally exhibits an opalescence or play of color like other opals but it does not need this to take a starring role in jewelry. Its juicy color is just the right accent to earth tones or black and also looks great paired with other bright tones.

Fire opals that do not show play of color are sometimes referred to as jelly opals.

Where are fire opals found?

Fire opals, with their mesmerizing play of color and fiery brilliance, are found in several locations around the globe, but some regions are particularly renowned for their deposits:

Mexico:

Querétaro: This central Mexican state is considered the "capital of fire opals." The famous Cantera Mine has produced some of the most spectacular stones ever discovered, known for their intense flashes of red, orange, and yellow.

Hidalgo: Another hotspot in Mexico, Hidalgo offers fire opals with a wider range of colors, including blues and greens. The La Gloria Mine is a notable source of these colorful beauties.

Australia:

Lightning Ridge: This town in New South Wales is another major player in the fire opal game. Here, you'll find opals with a unique play of color, often featuring patterns and flashes of blue, green, and even purple alongside the classic reds and oranges.

Honduras:

La Opalica Mine: This mine boasts the rare and coveted "black fire opals," which showcase their vibrant play of color against a dramatic, dark background. These gems are highly sought-after for their unique beauty and exclusivity.

Fire Opal from Mexico

Properties of Fire Opal

Composition: Microcrystalline quartz (chalcedony) with inclusions of iron oxide (hematite) and other minerals, which contribute to the fiery color. Higher water content (up to 10%) compared to other opal varieties gives it its unique properties.

Color: Primarily vibrant orange, red, and yellow, often with a play-of-color in similar warm tones. Some rare specimens may exhibit hints of green or blue.

Luster: Vitreous (glassy) to waxy, sometimes with a greasy sheen. Polish enhances the luster significantly.

Streak: White to light orange or reddish-brown.

Hardness: 5.5 - 6.5 on the Mohs scale (moderately hard, but requires caution due to brittleness).

Cleavage: Poorly defined.

Crystal Form: Usually found in massive or botryoidal (grape-like) forms, also as nodules and pebbles.

Density: 1.99 - 2.25 g/cm³ (lighter than most opals due to higher water content).

Transparency: Varies from translucent to opaque, depending on water content and inclusions.

Fracture: Conchoidal (shell-like).

Solubility: Insoluble in common acids and water.

Magnetism: Non-magnetic.

Fluorescence: May exhibit weak orange or yellow fluorescence under longwave ultraviolet light.

Uses of Fire Opals

Unlike most opal, fire opal is sometimes faceted, so you can choose sparkle as well as color. Because it is light as well as bright, fire opal is especially good for earrings, where even small sizes have a big punch of color.  Only the very finest qualities of Fire Opal are suitable for faceting. 

Fire Opal Care and Maintenance

Fire opals are relatively soft and can be scratched easily, so it is important to handle them with care.

Since fire opal has a high water content, it should be protected from heat and prolonged exposure to strong light, which could dry it out. Lapidaries cure fire opal by drying it before cutting to reduce instability, so it is actually not that delicate. But due to its relative softness, it is best suited for pendants, earrings, not everyday rings.

Ethiopian Fire Opal