Opal — Colour, Light and Varieties
Opal is a mineraloid composed of hydrated silica, with the chemical formula SiO₂·nH₂O. Unlike quartz, it does not have the ordered crystal structure of a single crystal. It occurs in a wide range of colours, from transparent and colourless to white, pink, blue, orange and black.
Some opals display an optical phenomenon called Play of Color: flashes of colour that change with lighting and viewing angle. Others do not display this phenomenon, and their beauty comes from bodycolour, transparency and natural pattern.
Gemological Properties
| Material type | Mineraloid — hydrated silica |
|---|---|
| Chemical composition | SiO₂·nH₂O |
| Structure | No single-crystal crystallographic system |
| Hardness | Approximately 5–6.5 on the Mohs scale |
| Specific gravity | Typical reference value around 2.15; varies with type, porosity and composition |
| Refractive index | Approximately 1.37–1.47, depending on type |
| Colour | A wide range, including white, black, pink, blue, yellow, orange and red |
| Transparency | Transparent to opaque |
| Optical phenomenon | Play-of-color in some varieties |
These are general reference values. In boulder opal, the accompanying host rock may affect the specific gravity measured for the complete stone.
What Creates Play-of-Color?
In opal displaying play-of-color, tiny silica spheres are arranged in a periodic structure. This causes diffraction and interference of light, producing flashes of colour. The size and arrangement of the spheres influence the colours seen.
Play-of-color is distinct from bodycolour. Black opal, for example, has a dark background against which the flashes appear, while fire opal can have an orange or red bodycolour without play-of-color.
Types of Opal
Common opal — without play-of-color
This group includes opal in colours such as white, pink, blue and brown that does not display play-of-color. “Common” describes the absence of this optical phenomenon and does not by itself determine quality or beauty. Pink opal, which has its own page on our website, belongs to this group.
White opal
Opal with a white or light bodycolour, against which multicoloured flashes may appear. Its pale background gives it a different appearance from black opal. White opal with play-of-color should be distinguished from white common opal without it.
Black opal
Opal with a dark bodycolour that need not be completely black. The background can also be dark grey and may emphasise contrast with the play-of-color. Quality depends in part on the brightness of the flashes, their pattern and their visibility from different angles.
Fire opal
Known for yellow, orange or red bodycolour, sometimes sufficiently transparent for faceting. “Fire” refers to its warm bodycolour: fire opal need not display play-of-color, although some stones combine both features.
Boulder opal
Opal cut together with part of the rock in which it formed. Opal veins and host rock create natural patterns and sometimes freeform shapes. The rock is a natural part of the stone, rather than a backing glued onto it.
Oregon opal
A name referring to geographic origin — Oregon in the United States — rather than a single uniform appearance. Diverse types have been documented at Opal Butte, including transparent opal, fire opal and opal with play-of-color, sometimes particularly noticeable when light passes through the stone.
Owyhee blue opal
A trade name for blue opal associated with Oregon’s Owyhee region. It is known primarily for its blue bodycolour, generally without play-of-color. The name refers to origin and appearance rather than a separate mineral family.
Sources Around the World
Australia is a major source of opal: Lightning Ridge is known for black opal, and Coober Pedy for white opal. Mexico is particularly associated with fire opal, and Ethiopia is an important source of opal with play-of-color. Oregon in the United States hosts diverse occurrences, including Opal Butte and the Owyhee area.
Treatments, Synthetic Opal and Assembled Stones
Opal may undergo treatments such as impregnation or darkening of its background through smoke or sugar treatment. Synthetic opal and imitations are also available.
Doublets and triplets are also sold: assembled stones made with a thin opal layer glued to a backing and sometimes a transparent top layer. They should be distinguished from solid opal and natural boulder opal, as construction affects value and jewellery care.
Cleaning and Care
Opal requires careful wear and protection from impacts, heat and sudden temperature changes. Clean gently with lukewarm water and a little mild soap, then dry with a soft cloth. Do not use steam or ultrasonic cleaning. For assembled stones or stones with unknown treatments, cleaning should be adapted to their construction.
Spiritual Meaning
Common opal — gentleness and emotional reflection
Opal without play-of-color is associated with gentle attention to emotions and creating a calm inner space. Its colours symbolise different aspects of this reflection: white is associated with clarity and inner cleansing, pink with tenderness and compassion, blue with calm and emotional expression, and brown and black with stability and connection to inner strength.
Oregon opal — joy and self-expression
Oregon opal is associated with imagination, openness and honest emotional expression. It symbolises willingness to recognise recurring patterns in our lives and make room for new choices. In this spirit, it is also linked to self-acceptance and finding joy through connection to personal wishes and values.
Fire opal — passion and creativity
Fire opal is associated with enthusiasm, vitality and the urge to create. Its warm colours symbolise connection to desire, enjoyment and freer self-expression. It can serve as a reminder to direct emotional intensity towards creative work and choices that express what matters to us.
Owyhee blue opal — quiet strength and clarity
Owyhee blue opal is associated with calm, confidence and clear expression. It symbolises a connection between inner listening and practical decisions: choosing words, setting a direction and acting with stability. Spiritually, it is also associated with introspection and the world of dreams and imagination.
White opal — awareness and openness
White opal is associated with heightened emotional awareness and willingness to examine personal patterns. It symbolises openness, letting go of habits that no longer serve us and making room for creative expression. Its play-of-color can serve as a reminder that a single experience may hold many perspectives.
Black opal — depth and personal intention
Black opal is associated with deep reflection, willpower and encountering less visible aspects of the inner world. It symbolises a search for clarity within complexity and directing emotion and intention towards a conscious personal process. In meditation, it is associated with inner listening and exploring memories and patterns.
These meanings belong to the realm of belief and symbolism. They can give jewellery personal significance, but they are not proven gemological properties or a promise of healing, protection or a change in health.