Filling is a gemstone enhancement method which indicates a broad range of treatments. From simple oiling to resin impregnation to the filling of fractures with glass or synthetic gem material with the aid of fluxes.
Oiling has been around since antiquity and is still one of the most commonly encountered gemstone treatments. It incorporates the filling of surface reaching fractures in both rough and cut stones with the aid of an oil. Applying oil, with a refractive index near that of the stone it's being applied to, hides the cracks in the gemstone and gives the stone the appearance of more color. Care should be taken when handling such treated gemstones as the oil can easily be removed by detergents or an organic solvent such as benzene. This treatment is not stable; the oil will dry out over time.
Resin impregnation is a relatively new technique which probably saw the light of day in the 1990's. Instead of oil an epoxy resin is used to fill the cracks. Resins are not just used to fill surface reaching fractures but also to stabilize and color porous materials such as opals, jade and turquoise.
Surface reaching cracks in rubies have been filled with lead glass from at least 1984. In this year the first filled stones were submitted to labs. This was colorless glass that had been used to fill surface cavities. The presence of gas bubbles in the glass filler proved an easy method to spot these fillings together with the difference of lustre of the filled areas versus the rest of the stone.
These days we see a lot of lead glass filled rubies which are produced from Mozambican low quality starting material. The filled areas can be spotted by the ‘blue flash’, the occaisional bubble and lustre difference. The glass used is susceptible to chemical wear when brought in contact with acids. Even lemon juice and vinegar will damage the glass after relatively short contact periods as you can clearly see in the image below:
In 1992 a new filling method presented gemologists with another challenge: the flux healing of sapphires. The new ruby find in Myanmar at Mong Hsu produced a lot of fractured crystals. Thai burners enhanced the unattractive appearance of these stones by first cleaning them with acids and then heating them in a flux with or without AlO2. This causes a ‘synthetic’ intergrowth of corundum to occur in the cracks which seals them. A very nifty treatment which aids the gemstone a lot, both in appearance and in durability. There are no known issues with this treatment in terms of durability.
They may serve as a good clue to the appraiser though. The flux healing almost always leaves flux feathers in the used-to-be cracks. The course nature and opaqueness of these flux remnants act as a readily identifyable feature which dates rubies to post 1992.
|Material||Effect||Date of introduction|
|Beryl (Emerald)||Clarity/color enhancement by (colored) oil or epoxy resin||Oil – BC
|Diamond||Glass filling||1982, Zvi Yehuda|
color and stability enhancement