Boulder opals are a valuable opal variety unique in their appearance, formation, and toughness. After black opal, Australian boulder opal is the most valuable variety. Boulder opals contain the same silica and water mixture typical of other varieties, but the opal forms in thin patches or seams of the boulder host rock — usually ironstone. Once cut, the result is a mesmerizing mixture of opal with a durable rock backing.

You can find many varieties of boulder opal for sale. Some have common opal (no play-of-color) while others contain precious opal (with play-of-color). The dark host rock of many Australian boulder opals creates a gorgeous backdrop for precious opal color-play — these are dubbed “boulder blacks.”

FAQs

What does a boulder opal look like?

Most Australian boulder opals have a foundation of dark rock hosting patches or seams of opal material, often iridescent-like precious opal. Boulder opals come in a wide array of patterns and colors, and the exact look depends on the type of boulder opal:

> Boulder Opal Wood Fossil: Opalized wood fossil attached to host rock
> Andamooka Painted Lady: Thin layer of opal (usually cool-toned in color) on host rock
> Yowah or Koroit Nut: Small, rounded Australian boulder opals (from Yowah or Koroit fields) resembling a walnut containing opal with distinct color banding or swirling
> Boulder Pipe Crystal Opal: Crystal opal in pipe or tube formation within host rock (usually sandstone)
> Cantera Opal: Pink rhyolite host rock containing precious opal or fire opal
> Boulder Matrix Opal: Opal spread throughout ironstone in a more intricate network of veins

Where do boulder opals come from?

Almost all boulder opals come from Queensland, Australia. Notable types of Queensland boulder opals for sale include boulder pipe crystal opals and Yowah nuts. Boulder opals also come from Canada, Brazil, Mexico, and Honduras.

How is boulder opal different from other types of opal?

Most finished opals are fully detached from their host rock, but boulder opals are still attached. The ironstone backing gives them greater durability and a sharper contrast for the precious opal’s play-of-color to shine.

For fashioning, Australian boulder opals are often cut into larger, freeform shapes while other opals are often cut into cabochons. The ironstone backing on boulder opals creates a natural doublet, which is stronger than a man-made opal doublet.

How hard is boulder opal?

Boulder opal hardness is usually around 6, while opals overall rank at 5 to 6.5. The host rock backing can boost the hardness — quartzite host rocks have a 7 to 7.5 Mohs hardness, for instance. Their durability is also better because they have less water, meaning lower risk of crazing.

What should I look for when buying a boulder opal?

The best boulder opals for sale have a dark host rock, few to no surface defects like cracks, and precious opal displaying a variety of vibrant colors. The amount of host rock should be minimal. Only buy from reputable sellers who are open to answering questions and providing documentation.

Can I get a certificate of authenticity with my boulder opal purchase?

Many of our Verified Sellers provide a certificate of authenticity with their boulder opals for sale. If they do not have one, you can message them to request one or use our Sheriff Program to have a third-party expert conduct an audit for authenticity and quality assurance.

How are boulder opals priced at auction?

It depends on many factors such as the stone’s quality and size, but boulder opals for sale at auction range from $10 to $8,000 per carat; most fall between $90 to $300 per carat. Some varieties like Yowah nuts may be pricier.

What payment methods are accepted for boulder opal purchases?

Every Verified Seller on Opal Auctions accepts GemPay, credit card, PayPal, and/or escrow. Our payment processor is PCI certified so all your card data is secure.

How is the boulder opal shipped after purchase?

Buyers have three days to pay after winning an auction. After that, the seller has 1-2 business days for processing. Shipping and delivery methods vary among sellers, and your location will influence the price and method of delivery, including potential customs fees. Visit the “extended profile” on the Verified Seller’s page to learn more about their shipping policies.

Can I return or exchange a boulder opal bought at auction?

Yes, all Opal Auctions sellers allow you to refund or exchange your boulder opal within 7 days of receiving it. Sellers may charge shipping and insurance rates for returns, so check their “extended profile” to review their specific policies.

How do I participate in a boulder opal auction?

First, register an account so you can track shipping and talk to sellers. Once you’ve found a boulder opal for sale, check if it’s an auction or non-auction listing by looking at the price area — prices on auction listings have a gavel with a “Place Bid” or “Make an Offer” button; prices on non-auction listings like Buy it Now items are green with an “Add to Cart” button.

For auction listings, click “Make an Offer” or “Place a Bid” and enter the maximum amount you’re willing to bid on the boulder opal. Make sure it’s above the reserve price set by the seller (unless it’s a No Reserve auction). If your bid wins, the seller accepts your offer and the item is added to your cart. Finish paying for the item and check your emails for shipping updates.

Unsure about committing to a bid? Add a boulder opal for sale to your “Item Watch” list to get email updates on bids so you can come back and bid later.

Further Reading