OPAL EDU
WHAT IS OPAL?
Opal is a hydrated amorphous form of silica (sio2·nh2o). When opal is formed, silica gel fills crevices in rock. As water evaporates, the silica is deposited in the form of tiny spheres. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
Mineral: hydrated silica
Color: all colors
Refractive index: 1.37-1.47
Specific gravity: 2.15 (+0.08, -0.90)
Mohs hardness: 5.5 to 6.5
Birthstone: October
Anniversary gift: fourteenth
National gemstone: Australia
There are two broad classes of opal: precious and common. Precious opal displays play-of-color, common opal does not. The internal structure of precious opal makes it diffract light, as the lightwaves travel between the spheres, the waves bend. Play-of-color occurs because opal is made up of sub-microscopic spheres stacked in a grid pattern. As they bend, they break up into the colors of the rainbow, creating the play-of-color. Depending on the conditions in which it formed, opal occurs in different colors. They vary in optical density- ranging from semi-transparent to opaque. Precious opal can be clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, black opal is the most rare, whereas white opal is the most common.
Opal can be found around the world, with Australian opal, specifically black opal regarded as the most precious and valuable.
Australia
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Solid (black, dark, light, crystal)
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Boulder
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Matrix
Ethiopia
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Solid (volcanic, hydrophanic)
Mexico
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Solid (fire opal / water opal)
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Boulder (fire opal matrix)
Brazil
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Solid (sedimentary light opal)
Peru
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Solid (blue and pink common opal)
Other sources: Canada, Czech Republic, Guatemala, Hungary, Honduras, Indonesia, and the United States
In 2008, NASA announced it had discovered opal deposits on Mars.
MAIN TYPES OF OPAL
Crystal / Water Opal
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Transparent to semi-transparent, with a clear background. Vivid and strong play-of-color.
White / Light Opal
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Translucent to semi-translucent, with play-of-color against a white body-color.
Black / Dark Opal
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Translucent to opaque, with play-of-color against a black or dark grey background.
Fire Opal / Mexican Opal
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Transparent to translucent, with red, orange, yellow, or brown body-color. May or may not show play-of-color.
Boulder Opal
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Translucent to opaque, with play-of-color against a light to dark background and rock matrix.
AUSTRALIAN OPAL
Opal is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The showers soaked deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward.
Production of precious opal began at White Cliffs, New South Wales in 1890, from Opalton, Queensland in 1896, and at Lightning Ridge, New South Wales in 1905.
Current major opal mining locations in Australia are the South Australian opal fields (Coober Pedy, Andamooka, and Mintabie), Lightning Ridge area, and Queensland.
Lightning Ridge produces the most valuable type of opal on the market. The high quality, rarity, and uniqueness makes black opal highly sought after for fine jewelry designers and investment collectors. This, however, creates challenges due to the fact that there are more buyers than mines, making gem quality opal scarce.
OPAL MINING FIELDS OF AUSTRALIA
figure source: GIA
LIGHT OPAL
BLACK OPAL
DARK OPAL
OPAL CLASSIFICATION
Classification based on transparency and base color.
Transparency: transparent (crystal opal), translucent (semi-crystal opal), opaque
Base color: black opal, dark opal, light opal
LIGHT
Crystal opal
White opal
SEMI-BLACK (DARK)
Semi-black / Dark crystal
Semi-black / Dark opal
BLACK
Black crystal
Black opal
BODY TONE
Translucent
Opaque
TRANSPARENCY
base body tone guide
According to the Opal Association, the base body tone refers to the darkness of the opal ignoring the play of color and brightness of the stone. The base color can only be determined by looking down on top of the stone and ignoring the material on the back of the stone. Only those stones ranking N1 to N4 are considered black opals - attracting additional value associated to this class.
BLACK OPAL
Black opal contains subtle traces of iron oxide and carbon elements, which are not found in common opal. These combined elements are responsible for creating the unique traits belonging to black opal. The dark canvas of black opal results in the distinction and reflection of much brighter colors compared to other varieties of opal. Yellow, blue and green are the most common colors seen through play of color, whereas violet, fiery red and orange colors are the most valuable and sought after of opal colors.
CRYSTAL OPAL
Crystal opal can be very translucent or ‘see through’ with some color or so full of color, you cannot see through them. The term crystal opal basically implies to any class of opal such as semi-black, white or black opal which has a translucent, lucid, or semi-translucent body.
BOULDER OPAL
Boulder opal combines precious opal with the ironstone in which it forms. Bright yellow, orange, or red fire opal are quite different from the other varieties of opal. Their day-glo tones, which are translucent to transparent, are beautiful with or without play of color. Most boulder opal comes from Queensland.
