gems, corundum, inclusions.5, Mozambique.12, ruby, böhmite

Gemologists at Bangkok’s Lotus Gemology laboratory recently examined a large lot of rubies believed to originate from the Montepuez area of Mozambique. Microscopic examination confirmed that assessment. Many of these stones displayed features in their infrared spectra that are typical for böhmite, with a characteristic “twin peak” spectrum at ~3309 and 3089 cm⁻¹. One of the stones showed a large area cut through on a pavilion facet that appeared to be a foreign substance. A similar substance was seen in many of the fissures in the stone. Judicious use of the hot point under the microscope produced no reaction. We then moved to analyze the substance via micro-Raman. The result was böhmite, which neatly confirmed the results from the infrared spectrum. Böhmite, AlO(OH), is a polymorph of diaspore. In corundum it is generally a secondary mineral, infilling surface-reaching openings after the host corundum has grown.

gems, corundum, ruby, Madagascar.12, inclusions.5

We have examined thousands of rubies in Lotus Gemology’s laboratory, yet the hours spent peering into the microscope are never mundane because each stone captures its own piece of history through its inclusions. Recently a client submitted one such gem for testing, a 3.04 ct specimen measuring 11.98 × 7.13 × 4.21 mm that was identified as an untreated ruby from Madagascar based on inclusions. In the microscope, a few bright brownish red crystal inclusions immediately stood out. They were transparent, with a saturated red hue that, in darkfield illumination, was conspicuous even against their ruby-red background. Some were also cut through on the surface, allowing us to view them with reflected light. Observation of the surface showed that the crystals had a luster just slightly lower than that of the surrounding corundum, suggesting a slightly lower RI. To get a better idea of the crystals’ identity, we examined them using micro-Raman spectroscopy with a WITec Alpha 300R Raman imaging system with a 532 nm laser, which identified them as staurolite. Staurolite, Fe₂Al₉Si₄O₂₂(OH)₂, is a red to brown mineral known to occur in Madagascar. Its RI values are nα = 1.736–1.747, nβ = 1.740–1.754, and nγ = 1.745–1.762. Corundum has a slightly higher RI range, with nₒ = 1.767–1.722 and nₑ = 1.759–1.763. The combination of microscopic observation and Raman spectroscopy gave us confidence that these were indeed staurolite crystals. It is no wonder these crystals initially stood out. To the best of our knowledge, this is the first recorded observation of staurolite as an inclusion in corundum. This Madagascar ruby has encapsulated a fascinating kernel of history within its depths.

gems, corundum, Madagascar.12, treatments.6

Madagascar has become one of the world’s top sources of fine blue sapphire in recent times. In addition to beautiful untreated material, increasing numbers of treated stones have appeared in the market. Some have been heated to relatively low temperatures, below 1350°C, to lighten their color. To help separate unheated and heated Madagascar sapphire, the authors performed experiments to document the changes they undergo with low-temperature heat treatment in air, which is an oxidizing atmosphere.

gems, corundum, synthetic corundum.7, Verneuil synthetic sapphire

Most synthetic ruby and sapphire on the market is grown by Verneuil flame fusion. It can usually be separated from natural corundum by its distinctive curved banding, in contrast to the angular zoning seen in natural stones. Gemologists may see these features in the microscope when using darkfield or brightfield illumination. This zoning can also be seen with use of a short-wave fluorescent light, as noted in Ruby & Sapphire: A Gemologist’s Guide (R.W. Hughes et al., Lotus Publishing, Bangkok, 2017). Recently the author noticed two excellent examples. When viewed with the Magilabs deep-UV fluorescence system (a proprietary short-wave UV source), the curved banding in the synthetics was clear (figures 10 and 11), allowing them to be easily separated from natural corundum. Gemologists using a DiamondView may see the same reaction.

gems, corundum, inclusions.5, color.4

In the second of a four-part series for Gems&Jewellery, Bangkok-based gem testing lab, Lotus Gemology, shares insightful pairs of photographs that, when placed side-by-side, reveal the full story of an inclusion or treatment. In this issue, we explore heat-treated sapphires...

gems, corundum, inclusions.5, rutile silk, primary rutile

In the final part of a year-long series for Gems&Jewellery, Billie Hughes of Bangkok-based gem testing lab, Lotus Gemology, shares pairs of photographs that, when placed side-by-side, reveal the full story of an inclusion or treatment. In this issue, we delve deeper into rutile inclusions in ruby and sapphire...

gems, corundum, inclusions.5, Mozambique.12

Lotus Gemology receives submissions of star stones on a regular basis, so when we first saw the specimen shown here, it appeared to be one among many of our usual stones to test. Upon closer examination, however, we realized it was not a star sapphire, but something much more rarely seen: star spinel. At first glance, the the cabochon’s dome displays one six-rayed star (figure 16), similar to what we see in star corundum. But if we follow each ray to the sides of the stone, we can also see a four-rayed star (figure 17). These stars are created from numerous dense, short needle-like particles. Once we examined a small polished area on the cabochon’s base, we found that there are actually two distinct types of exsolution products in the stone. On the right side of figure 18, we can see the tiny, dense needles and particles that form both types of stars. In the center of the image we can see much larger, less densely dispersed platelets that resemble confetti.

gems, corundum, inclusions.5, Burma.12

While testing a faceted pink sapphire in our laboratory, we looked at its internal features and determined that it was a heat-treated stone from Myanmar. As we examined the surface of the 14.75 × 13.02 × 8.82 mm sapphire, we noticed an unusual feature: a large round drill hole extending from the surface into a crystal or negative crystal within the stone (figure 4). Note that this drill hole did not extend through the sapphire, as one might see in a bead. This stone was faceted, so it is unlikely that the person who drilled into the stone intended to create a bead. What seems more likely, given the placement of the drill hole, is that it was drilled to minimize the appearance of the crystal that it reached. We suspect that the crystal was originally more opaque and of darker color or filled with a dark substance. It is possible that the drill hole was created so that the crystal could be cleaned out with acid, minimizing its appearance. We can only guess at the intentions of the treater, who might have gotten the idea from the laser drilling of diamonds. Although the exact process may remain a mystery, this inclusion indicates an unusual treatment in ruby and sapphire.

corundum, inclusions.5, Burma.12, gems

The Mogok Stone Tract in Myanmar (formerly Burma) is one of the world’s most famous sources of ruby, which often forms there in a calcite-marble host. Calcite may occur as an inclusion inside the ruby, as in the example in figure 3. In this stone, the calcite is easy to spot between crossed polarizers, which reveal polysynthetic twinning planes that “crisscross” the crystal. While calcite is a common inclusion in ruby, this is the clearest example of calcite twinning this author has seen. Surrounding the calcite crystal is a dense, angular nest of exsolved rutile silk as short needles, a typical scene in rubies from this locality.

gems, sapphire, corundum, Burma.12

A survey of the most important sapphire deposits around the world.