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Hi everyone,
We at Navneet Gems are hoping to learn from the community’s experience with both Australian and Montana sapphires. We’d love to hear your thoughts on the similarities and differences between the two origins.
A few questions we’re curious about:
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What stands out to you about Montana sapphire color vs Australian sapphire color?
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Is it fair to say that parti sapphires (often called “pharaoh’s eye”) are the signature look of Australian sapphires, while teal tones are the hallmark of Montana stones?
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In terms of market value, what kind of price difference do you see between top-tier Montana vs Australian sapphires of similar quality around the 1 ct mark? Are we talking 20%, 30%, 50%?
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And from those familiar with both regions: how would you compare the overall supply and availability?
We’re simply looking to gather perspectives from people who work with or collect stones from both origins.
For anyone interested, here’s an example of something you rarely see in Montana material — an 11.61 ct stone we call the Star of Australia: https://www.navneetgems.com/introducing-the-star-of-australia/
We’ve been in the trade since 1993, but it’s always great to hear insights from Australian cutters and collectors. Looking forward to the discussion!
Feature Gems & Gemology, Winter 2015, Vol. 51, No. 4
The Origin of Montana’s Alluvial Sapphires
J.C. (Hanco) Zwaan, Eric Buter, Regina Mertz-Kraus, and Robert E. Kane
The above article from the GIA. It traces the origin of alluvial sapphires in Montana. These sapphires are of metamorphic/metasomatic orgin. Plumasite pegmatitic sources are implicated based on low Fe, low Ga/Mg ratios as well as mineral inclusions. Yogo Gulch sapphires were brought to the surface by a lamprophrye. These sapphires are magmatic/basaltic. Low iron, high Ga/Mg ratios, hi titanium. Alluvial sapphires are pale with silk. The low iron content makes them paler than Australian magmatic sapphires.
Australian Central Queenlands Gemfield sapphires were brought to the surface by voluminous Cenzoic alkalic basaltic eruptions. These are magmatic sapphires with high iron and high Ga/Mg ratios. The deeper blues, greens and increased clarity are the result of a high iron content, less titanium. The sapphires formed as xenoliths from high aluminum deep crustal strata, brought up from the depths by alkali basalt eruptions. Sappires from Montana are small, with cut stones being less than 1 ct… Australian sapphires are larger with cut stones exceeding 1-2 ct.
The value of both types of sapphires of top quality are equally as high. Montana sapphires are less abundant than Australian. Their relative rarity and limited supply give them value.
Really interesting! Montana sapphires are mostly alluvial and pale due to low iron and high titanium, and usually small (<1 ct), while Australian sapphires are magmatic, deeper-colored, and larger. Both can be top-quality, but Montana’s rarity gives it unique value.
I agree with you on that. Montana sapphires are low volumn finds. Yogo gulch is still producing but at much lower scale than in the past. Alluvial sapphires are hard to find. Top quality australian ones of larger carat size and clarity still have much value. Kashmiri sapphire deposits have been completely dug out. Those sappphires are metamorphic from desilicated plumasite pegmatites. Mogok region gems formed in limestone to marble metamorphic environment. The aluminum came from pelitic clays and high aluminum content shales.. following the continental collision between India and Asia, which transformed the impure limestones into marble, the area was intruded with syenites and other low silica activity alkalic rocks. Marble is stable across a wide range of metamorphic conditions. Marble allows for an efficient mechanism to desilicate igneous instrusions, forming corundum. Chromium comes from igneous rocks.. rubies are formed.. where the chromium is absent, sapphires form. the jadeite there is from a metamorphosed ultrabasic suite of rocks- an ophiolite, an obducted slab of oceanic floor rock. The rock is under silica saturated. Jadeite itself is undersaturated. Omphacite is neither. It depends on the host rock. However, omphacite is also found in association with Jadeite. Sapphires from Cambodia and Vietnam are mostly of igneous origin, like Australian. Thai deposits have also been depleted. The best indicator of basaltic versus metamorphic sapphires is the Gallium to Magnesium ratios, with igneous/basaltic sapphires having ratio of 10 or>, metamorphics are <10.. there is considerable overlap between the two types. Other isotope ratios, oxygen can be another discriminator, along with the trace element profile.. Iron is ubiquitous in corundum. Titanium is an incompatible element in the crystal structure. Iron and Ti, replace aluminum. The blue color of sapphire results from the charge exchange between Fe+2 and Ti+4…All of these analytic methods require LA-ICP-MS… the geology of gem deposits and geochemical analysis of gemstones is a clue to their origin down to provenance.
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Totally agree—Montana sapphires are rare, Yogo Gulch is still producing but on a smaller scale, and alluvial stones are hard to find. Kashmiri sapphires are mostly gone, Mogok stones grew in marble, and rubies form where chromium is present. Cambodian, Vietnamese, and Australian sapphires are mostly igneous, and Ga/Mg ratios plus trace elements can help tell metamorphic vs. igneous stones apart. Blue color comes from Fe²⁺ ↔ Ti⁴⁺ charge transfer, and techniques like LA-ICP-MS can often trace a gem’s exact origin.
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yogo gulch sapphires are in a lamprophyre. The rock is igneous. Lamprohyres have many varietal names depending the minerals contained within. These are ultrapotassic unsaturated rocks with a deep origin, at the mantle transition zone, 410 to 660 km below the earth’s surface. When the Farallon plate subducted at a low angle, it dropped into the mantle transition zone. Volatiles and Aluminum “fertilized” the MTZ melt and provided for the sapphires at Yogo.Gulch. Other alluvial sapphires from Montana are metamorphic sapphires.
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