Opal: Play of Color and Formation

## Opal Is Different Opal breaks almost every rule of conventional gemology. It is not crystalline — it is amorphous silica with no repeating atomic structure. It is not hard — at 5.5–6.5 on Mohs, it is softer than glass. It contains water — typically 3–20% by weight, which makes it sensitive to dehydration. And its defining optical feature — play-of-color — is caused by a physical structure (diffraction) rather than a chemical coloring agent. Despite all these apparent disadvantages, fine opal is one of the most visually spectacular gemstones in existence. A top Australian black opal displaying broad flashes of red, green, and blue across a dark body is among the most mesmerizing objects in the mineral kingdom. ## How Play-of-Color Works Opal is composed of tiny spheres of silica (SiO₂) arranged in a three-dimensional grid. When these spheres are uniform in size and regularly stacked, they act as a diffraction grating — white light entering the stone is split into its component spectral colors, which flash from different angles as the stone moves. The size of the silica spheres determines which colors appear: - Smaller spheres (approximately 150 nm) produce violet and blue - Medium spheres (approximately 200 nm) produce green - Larger spheres (approximately 300+ nm) produce orange and red This is why red play-of-color is the rarest and most valued — it requires the largest uniformly sized spheres, which are geologically less common. An opal that displays red play-of-color also shows all the colors below red in the spectrum. ### Types of Opal **Precious opal**: Displays play-of-color. The silica spheres are orderly enough to diffract light. **Common opal (potch)**: No play-of-color. The spheres are too irregular or randomly sized to produce diffraction. Common opal can be attractive in its own right — fire opal (transparent orange, from Mexico) and Peruvian blue opal are examples. ## The Opal Varieties ### Black Opal The most valuable variety. "Black" refers to the dark body tone (dark gray to black) against which play-of-color appears with maximum contrast and visual impact. Lightning Ridge, New South Wales, Australia, is the world's premier source. Fine black opal with broad-pattern red-on-black play-of-color can exceed $10,000 per carat. The darkness of the body comes from carbon and iron oxide particles distributed throughout the silica structure. Not all Lightning Ridge opal is black — the deposit also produces dark, semi-black, and crystal opal. ### White Opal Light-bodied opal (white to light gray body tone) with play-of-color. Coober Pedy, South Australia, is the most famous source. White opal displays softer, more pastel play-of-color because the light body provides less contrast. It is more abundant and less valuable than black opal — fine material ranges from $100–$1,000 per carat. ### Crystal Opal Transparent to semi-transparent opal with play-of-color. The transparency allows light to enter the stone deeply, creating a three-dimensional display. Crystal opal with a dark body tone ("black crystal") is particularly prized. ### Boulder Opal Thin veins of precious opal formed within ironstone matrix. The ironstone provides a natural dark backing that enhances play-of-color (similar to black opal's dark body). Queensland, Australia, is the primary source. Boulder opal is cut with the ironstone matrix attached, creating unique, often freeform shapes. ### Fire Opal Transparent to translucent opal with a vivid body color — orange, red, or yellow — typically without play-of-color. Mexico (Queretaro) is the classic source. Fire opal is faceted like conventional gemstones, displaying brilliance from light refraction rather than diffraction. The most valued fire opals combine vivid orange body color with play-of-color — a combination that is rare. ### Ethiopian Opal Discovered in Welo Province in 2008, Ethiopian opal has reshaped the market. The deposit produces crystal to white opal with vivid, broad-pattern play-of-color at prices well below Australian equivalents. Ethiopian opal tends to be hydrophane — it absorbs water, which can temporarily change its appearance. This hydrophane property is a mixed blessing: it makes the stones somewhat less stable than Australian opal. ## Formation and Geology Opal forms when silica-rich water percolates through sedimentary rock and deposits silica in cavities, cracks, and fossils. Over millions of years, the silica spheres settle into ordered or disordered arrangements. **Australian opal**: Forms in Cretaceous sedimentary basins. The opal fills cavities in sandstone and claystone, sometimes replacing fossils (opalized shells, bones, and even complete marine creatures). The Australian deposits are the oldest and most geologically stable, which is why Australian opal tends to be the most durable. **Ethiopian opal**: Forms in volcanic rhyolite deposits. The different geological origin explains the hydrophane property — the volcanic host rock creates a more porous silica structure. **Mexican fire opal**: Forms in volcanic rhyolite, similar to Ethiopian opal but in older volcanic deposits. ## Evaluating Opal ### Play-of-Color Pattern Pattern — the way play-of-color is distributed across the face of the stone — is a critical value factor: - **Harlequin**: Large, angular patches of color in a checkerboard pattern. The rarest and most valued pattern. - **Broadflash**: Large areas of single color that flash as the stone moves. - **Rolling flash**: Color that sweeps across the face in a single wave. - **Pinfire**: Tiny points of color. Less valuable than broad patterns but can be attractive. - **Straw**: Thin, parallel lines of color. Generally less desirable. ### Color Red is the most valued play-of-color, followed by orange, then green, then blue. A stone showing all spectral colors ("full spectrum") is more valuable than one showing only blue-green. ### Body Tone Darker body tones provide better contrast for play-of-color. Black opal's value premium reflects this directly. ### Brightness How vivid and brilliant the play-of-color appears — from dull (barely visible) to brilliant (blazing flashes visible from any distance and angle). ## Care and Stability Opal requires more care than most gemstones: - Avoid sudden temperature changes (thermal shock can cause crazing — a network of fine surface cracks) - Store with a slightly damp cloth nearby in dry climates to maintain humidity - Avoid ultrasonic cleaners and steam cleaning - Clean with mild soap and lukewarm water only - Remove opal jewelry before physical activity - Ethiopian opal should be kept away from prolonged water immersion (hydrophane effect) Despite its fragility, well-cared-for opal can last generations. Many Victorian and Art Nouveau opal pieces survive in excellent condition over a century later.