GEMSTONE SPECIES - INORGANIC

“Opal” comes from the Latin Opalus, which likely came from the Sanskrit word Upala, meaning “precious stone.” Opal is best known for OPALESCENCE, its play-of-colour, a phenomenon caused by the diffraction of light within tiny silica spheres inside the stone. This effect makes opal shimmer in various hues like red, green, blue, and violet, often all at once.
Opal is the national gemstone of Australia as it produces over 90% of the world’s precious opal, including the most valuable types like black opal, boulder opal, and white opal. The most famous and valuable opal sources are Lightning Ridge, Coober Pedy, Andamooka, and Quilpie, are located in Australia.
Opals are a hydrated amorphous form of silica. The range of water content in precious opal is 10%, and others minimum of 3%. Opal sometimes crazes and cracks due to dehydration due to loss of water. It can occur in the fissures in any type of rock and is deposited at a very moderate temperature or cooling of igneous rocks along limonite, sedimentary rock, and rhyolitic lava domes.

TYPES OF OPAL & VALUE FACTORS:
--> Black Opal: Dark body tone (black to dark grey) best contrast for play-of-colour
Typical Colour: Vivid rainbow flashes
Main Source: Australia (Lightning Ridge)
--> White Opal: Pale body tone with soft play-of-colour
Typical Colour: Light blues, pinks, greens
Main Source: Australia (Coober Pedy)
--> Boulder Opal: Opal formed within ironstone rock often has irregular shapes
Typical Colour: Bright flashes with dark matrix
Main Source: Australia (Queensland)
--> Crystal Opal: Transparent to translucent with intense play-of-colour
Typical Colour: Various
Main Source: Australia, Ethiopia
--> Fire Opal: Transparent to translucent with fiery red, orange, or yellow body colour
Typical Colour: May or may not show play-of-colour
Main Source: Mexico
--> Common Opal: No play-of-colour (also called “potch”)
Typical Colour: Milky white, pink, green, etc.
Main Source: Worldwide
--> Ethiopian Opal: Often hydrophane (absorbs water)
Typical Colour:  vivid colours and patterns, Rainbow flashes in jelly-like body
Main Source: Ethiopia (Welo)

VALUE FACTORS OF OPAL:
1. Play-of-Colour (Primary Factor)
--> The most important value factor for precious opals.
--> Brighter, broader, and more varied colours have higher value.
--> Red is usually most valuable, followed by orange, green, and blue.
2. Body Tone
--> Black or dark opals are generally more valuable than white or light opals.
--> A darker background enhances the colour contrast.
3. Transparency
--> Crystal opals (transparent/translucent) with vibrant colour are highly prized.
--> Milky or cloudy opals may be less valuable unless a colour is strong.
4. Pattern
--> The arrangement of colour is important.
--> Rare and beautiful patterns like harlequin, pinfire, broad flash, or rolling flash are highly sought after.
5. Brightness
--> Brighter play-of-colour is equal to more value.
--> Dull or washed-out colours reduce value.
6. Cut and Shape
--> Symmetry and polish matter.
--> Cabochons (domed shapes) are most common and preferred.
--> Freeform cuts are often used for boulder opals.
7. Size and Carat Weight
--> Larger opals are rarer and more valuable, especially with high colour quality.
--> Thin slices or doublets are less valuable than solid opals.
8. Origin
--> Lightning Ridge (Australia): Highest quality black opals.
--> Ethiopia (Welo): Beautiful but often hydrophane and more fragile.
--> Mexico: Fire opals with or without play-of-colour.

IMITATIONS AND TREATMENTS
--> Doublets and triplets: Assembled stones with thin opal layers are less valuable.
--> Synthetic opals: Lab-made (e.g., Gilson opals) recognised by uniform colour patterns.
--> Hydrophane opals: May absorb water, temporarily changing appearance so oil treatment & opticon to        improve transparency while preserving the play of colours,
--> Dying, Sugar-acid treatment: To improve durability & transparency
--> Resin Treatment: Used to stabilise opals, particularly those prone to cracking & crazing
--> Filling Fractures & inclusions: Options commonly used on valconic & some boulder opals

--> Smoke Treatment or heating the opal to create a darker background and enhance the play-of-colour.

Disclosure of Opal Treatments – What Buyers Should Know
When it comes to opal gemstones, it's essential to disclose any kind of treatments that alter the gemstone's appearance or stability. Some treatments are accepted in the trade, while others significantly affect value. Full transparency is important for ethical selling and informed buying.