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Boulder Opal Gemstone Information

Boulder Opal Gemstones from GemSelect - Large Image
Buy Natural Boulder Opal from GemSelect

About Boulder Opal - History and Introduction

Boulder opal is one of the most valuable varieties of opal, second only to Australia's black opal. Fine specimens of boulder opal can exhibit all of the spectral colors through their distinct play of color. Boulder opal may be considered to be less valuable than black opal, but it is actually much rarer. Boulder opal accounts for approximately 2% of all of Australia's opal, however, black opal equates to around 8% of the entire yield. The remaining 90% is common opal, often referred to as 'potch' or 'white opal'. Australia produces over 95% of the world's entire supply of opal gemstones, clearly establishing it as the world's leading opal supplier. Boulder opal was first discovered in Quilpe, Western Australia, around the year 1870. Almost all boulder opal available today is from Western Queensland's opal-rich fields, confined to an area of about 200 to 300 square kilometers.

Boulder opal is found embedded in large boulders of ironstone, which is how it earned its name. Occurring as pebble rock, precious opal develops within thin veins, fissures and hollows. As opal miners attempt to remove the embedded opal from the ironstone, some of the host matrix is preserved during the process. For this reason, boulder opal is sometimes referred to as 'opal in matrix'. The ironstone matrix enhances the stones durability and vibrancy of color, often increasing the desirable effects of 'play of color' and 'opalescence', which are two different optical phenomenona, commonly mistaken for being one and the same. Boulder opal is similar to layered opal doublet, but rather than being assembled, boulder opal's layering is a natural occurrence.

Compared to other varieties of opal, boulder opal has a much higher density because of its ironstone matrix. As a result, boulder opal precious gems are much smaller in size compared to common opals of equivalent weight. Boulder opal is often cut in irregular, free-form shapes, in attempt to maximize the size and value of the finished opal stone. As a testament to boulder opal's significance and popularity in the gem trade, it is one of the official birthstones for the month of October, along with all of the other varieties of opal gems.

Boulder Opal

Identifying Boulder Opal

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Boulder opal is easily distinguished by its solid brown ironstone matrix. The ironstone is very dark and it enhances play of color, much like potch on the backside of black opal. Boulder opal's thin layer of precious opal is easily identified by its amorphous crystal structure and distinct optical features.

Unlike other gem types with play of color or iridescent effects, opal also exhibits 'opalescence'. Opalescence is caused by the reflection of light, and typically appears in a milky bluish color. Other iridescent stones do not display this pearl-like bluish color effect that appears to glide across opal. Other iridescent stones have color that will abruptly appear, disappear and then reappear, depending on the viewing angle. Close observance of opal's color phenomena can usually help distinguish it from any other similar looking gems.

Boulder Opal Origin and Gemstone Sources

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Australia is the world's only source for boulder opal and it is the leading opal gemstone supplier for other varieties as well. Slovakia was once the number one provider of opal, but mines have long been worked out. Other types of opal gems can be found in various locations around the world, but boulder opal is specific to Australia, primarily Western Queensland.

Buying Boulder Opal and Determining Boulder Opal Gemstone Value

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Boulder Opal Color

Boulder opal can exhibit a range of colors as a result of its distinct play of color. All the colors of the spectrum are possible, but blue, green, yellow and orange colors are the most common. Some opal may also exhibit 'opalescence', which is typically a milky, whitish blue color, but it can also occur in other colors as well. High quality boulder opal can exhibit all colors of the rainbow in a single specimen. The primary body color of boulder opal is usually a dull brown, gray or black.

Boulder Opal Clarity and Luster

Boulder opal is an opaque gemstone due to its ironstone content. The thin strip of precious opal can range in clarity from a rare transparent material to a more common, translucent material. Ironstone remnants can often be visible through the surface of the opal, affecting its clarity and giving it an unattractive spotted appearance, making those with no visible spotting most desirable. The top layer of boulder opal has a waxy or resinous luster, often described as 'plasticky'.

Boulder Opal Cut and Shape

Boulder opals are mostly found in free-form shapes and slabs that maximize and preserve the weight of the precious opal strip. They are most often cut into low domes, or flat cabochons, but the final shape truly depends on the thickness of the opal's color bar. If the opal color bar is thin, it will be cut flat or with a low dome. It is not uncommon for boulder opal to have an undulating, wavy surface as well.

Boulder Opal Treatment

Boulder opal is not typically treated in any way. They are naturally layered with host rock matrix. Boulder opal is often referred to as 'natural opal doublet', because of its shared similarities with opal doublet. However, opal doublet gems are assembled, whereas boulder opal is naturally layered with its host rock matrix.

Boulder Opal Gemological Properties:

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Chemical Formula: SiO2_nH2O Hydrous silicon dioxide
Crystal Structure: Amorphous
Color: All colors - play of color
Hardness: 5.5 to 6.5 on the Mohs scale
Refractive Index: 1.37 to 1.52
Density: 1.98 to 2.50
Cleavage: None
Transparency: Opaque, translucent, transparent
Double Refraction or Birefringence: None
Luster: Waxy - resinous
Fluorescence: None

Please refer to our Gemstone Glossary for details of gemology-related terms.

Boulder Opal: Varieties or Similar Gemstones:

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Opal Doublet
Opal Doublet

Opal is hydrated silicon dioxide, which means it has an extremely high water content and an amorphous crystal structure. There are many different varieties of opal with some being extremely rare and valuable. Boulder opal is considered to be the second most valuable opal and one of the rarest as well. Boulder opal is very similar to opal doublet, as they are both layered opals. Because of its obvious layer of brown ironstone, very few gems could possibly be confused with boulder opal. Some possibilities for confusion include organic fossilized ammolite, labradorite and maybe fire agate.

Most Popular Similar or Related Gems:

Precious opal, black opal, white opal, fire opal, common opal (potch), chocolate opal, opal doublet, opal triplet and opal in matrix are the most popular similar or related gems.

Lesser-Known Similar or Related Gems:

Crystal opal, black crystal opal, Mexican opal, sun opal, harlequin opal, jelly opal, matrix opal, agate opal, angel skin opal, girasol, honey opal, hyalite, hydrophane opal, porcelain opal, prase opal, moss opal, wax opal and wood opal are the rarer or lesser-known varieties of similar or related gems.

Boulder Opal Gemstone Mythology, Metaphysical and Healing Powers

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Boulder opal is one of the many varieties of opal, carrying a great deal of gemstone lore and legends. 'Opal' comes from the Greek word 'opallus', which loosely translates as 'changing color'. The world 'opallus' slowly evolved into 'opalus', a Latin word expression meaning 'precious stone'.

Boulder opal is thought to help alleviate depression and help find true love. Since boulder opal can display a wide array of colors, it possesses a range of healing abilities. Physically, it can ease pain and suffering associated with the abdomen, lower back and intestinal organs, including the liver, kidney and adrenal glands. Boulder opal is also believed to help reduce symptoms caused by the common cold and influenza.

Boulder opal is October's official birthstone. Boulder opal is also the zodiac stone for Cancer, Libra, Pisces and Scorpio, and it is the astrological gemstone for the sign of Virgo. Boulder opal is assigned to the element of fire, although it can enhance all of the elements. Boulder opal channels a projective energy that can benefit all chakras.

Disclaimer: Metaphysical and Alternative Crystal Healing Powers and Properties are not to be taken as confirmed advice. Traditional, Ceremonial and Mythological Gemstone Lore is collected from various resources and is not the sole opinion of SETT Co., Ltd. This information is not to replace the advice of your doctor. Should you have any medical conditions, please see a licensed medical practitioner. GemSelect does not guarantee any claims or statements of healing or astrological birthstone powers and cannot be held liable under any circumstances.

Boulder Opal Gemstone and Jewelry Design Ideas

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Opal is one of the softer gem types and it is considered to be quite delicate compared to most other gems. However, as a result of its ironstone composition, boulder opal is much more durable than other opal, making it much more suitable for jewelry use.

Boulder opal is best worn in protective style jewelry that is less prone to hard knocks and blows, such as earrings or brooches. Boulder opal is best used for pendants because of the availability of unusual and interesting free-form shapes. It is not very common to find boulder opal set into bracelets or gemstone rings, because boulder opal stones are very flat with very low domes, which is not the ideal form for bracelets or rings. All opals in general are not ideal for everyday jewelry, but for occasional-wear, boulder opal can make excellent, high-end fashion jewelry.

Note: Buy colored gemstones by size and not by carat weight. Colored stones vary in size-to-weight ratio. Some stones are larger and others are smaller than diamond by weight in comparison.

Boulder Opal Gemstone and Jewelry Care and Cleaning

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How to Clean your GemstonesBoulder opals are extremely fragile and delicate gemstones. If boulder opal jewelry is not properly taken care of, the layer of precious opal can crack and fade, which is known as 'crazing'. Opals are fairly soft compared to other gemstones, making them extremely vulnerable to scratches and loss of polish. Although boulder opals are slightly more durable, they do still require a high level of care. It possible, store your boulder opal gems and jewelry in a dehumidified room, especially if you live in a dry climate. When storing opal gemstones for extended periods of time, place them inside a tight plastic bag with some type of moisture source, such as a damp tissue or cloth. This will prevent your precious opal from dehydration. Boulder opal is very sensitive to sudden temperature fluctuation, so try to keep boulder opal gemstones in a stable environment.

Opal gemstones and jewelry should be cleaned on a regular basis. You can use mild soap, warm water and a soft cloth to wipe them down. Since opal is quite porous, you should also avoid the use of bleach, chemicals, cosmetics or household cleaners when wearing opal jewelry. Do not use ultrasonic cleaners or steamers and always remove jewelry before engaging in vigorous physical activity, such as sports, exercise or household chores. Damaged boulder opals are near impossible to repair, so in most cases, it is far better to replace damaged stones, rather than trying to repair them. However, many opals have been repaired by way of impregnation of polymer, wax or resin, but this is not an officially recognized or approved method for repairing opal gemstones.

  • First Published: August-27-2013
  • Last Updated: January-07-2019
  • © 2005-2019 all rights reserved.
    Reproduction (text or graphics) without the express written consent of (SETT Company Ltd.) is strictly prohibited.
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