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boulder opal - freeform & fabulous
The world's richest boulder opal mine is considered to be Hayricks, near Quilpie, in the magnificent Mt. Canaway area of the Queensland Opal Belt.
When opal is found as a thin layer on ironstone, exposed on one side while the natural stone is left on the back, the result is boulder opal . When the opal is mixed through the ironstone it is called matrix boulder opal .
A boulder opal which has a 'full face' with no ironstone intrusions is said to be 'clean-faced' and commands a higher price.
In general, opal with red fire is the rarest and most valued because it will also show other colors when rolled back and forth.
Boulder opal has no association with Boulder, Colorado, as many of the uninitiated think when first seeing it or hearing about the stone.
Boulder opal is a solid opal as opposed to a doublet (a thin layer of opal glued to a backing of black potch, glass or Queensland ironstone) or triplet (a flat thin slice of opal glued to a backing and covered with a protective quartz dome).
A yowah nut is a form of boulder opal from Yowah (Queensland), Australia, which occurs most often as a nut-sized ironstone nodule containing pockets and veins of vivid precious opal.
 the boulder belt

queensland opal territory
map: courtesy of len cram

Discovered around the turn of nineteenth century, boulder opal is a variety of precious opal that occurs intermittently within a belt of cretaceous sedimentary rocks from the New South Wales' border to about 600 miles north-west -- an area called the Queensland (the "Boulder belt").
A vast arid region, Queensland is the source of over 90% of all the world's precious opal, including boulder opal.

matrix boulder opal

Found in veins and cavities of ferruginous sandstone ("ironstone"), boulder opal occurs as thin veins of precious opal in the cracks and cavities of light and dark brown ironstone boulders in Queensland.
Defined as "presented in one piece, where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition," boulders can range in size from a table to a small potato. But whatever size, they have one thing in common: when split, they reveal a layer of vibrant opalescent colors which only increase in beauty as the stones are shaped and polished.
opal-essence
The physical structure of precious opal is unique. Its magical "opal-essence" is a result of the 5-10% of water trapped inside the stone in which rows of tiny silicon dioxide spheres are arranged, diffracting light in a unique fashion.

The spheres form a pyramid shaped grid, interspersed with water -- and it's the tiny natural faults in this grid that cause the characteristic "play of color."
Many experts have declared Queensland boulder opal to be the brightest and best appearing of all the Australian opals.
Because of its weighty ironstone back, boulder opal, unlike most precious gems, is sold by the piece rather than the carat.
When opal-faced ironstone was first discovered, the opal portions were generally removed from the matrix and polished (leaving a thin fragile layer of opal). Since the 1980s, however, boulder opal has been polished following the shape of the mother rock, leaving the matrix layer intact.
Individual stones are usually under $1,000 (though connoisseur specimens are now commanding $5,000 to $50,000).
Opal mining is said to be about patience and profits. But there's often little correlation between the two.
 layered magic
Famous worldwide for it's stability, strength, uniqueness and vibrant colors similar to those found in black opal, boulder opal's 'gemmy' color layer consists of a facing -- a thin sheet of opal that naturally adheres to and overlays the ironstone -- which has the effect of making boulder opal more durable than solid opal.

'crazed' precious opal with fine cracks

The ironstone backing not only strengthens the opal, it can give it a more interesting appearance, sometimes described as an undulating surface of "hills and valleys." As Fred Ward says in his Gem Series book "Opals":

"Neither flat, nor faceted, nor domed like its black and light counterparts, boulder opal undulates across the ironstone face as water flowed there millenia ago....endowed with beauty and durability..."

And, because water content within boulder opal is lower than solid opal, it almost never cracks or crazes (develops fine cracks) as it ages, as some other kinds of opals -- making it the most durable of the opals.

carved seal, boulder opal

Cut into unusual or freeform shapes and polished on the gem side, the strong natural ironstone back is retained to form part of the gem. In recent years the unusual shapes and ironstone inclusions particular to boulder opal have made it increasingly popular with trend-setting designers and wearers.
Boulder opal really is a bargain: it has an appearance very similar to "black opal" (the most expensive), yet is but a fraction of the price.

symbolic attributes

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modern October
traditional April
ayurvedic October
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