Opal crazing : understanding to better prevent
Opal, a fascinating gemstone with its play of colors and translucency, is also sadly renowned for its fragility. Crazing, and sometimes bleaching, compromise the aesthetic and commercial value of many opals, particularly those from volcanic deposits such as those in Ethiopia or Mexico. This phenomenon has long been poorly understood, combining empirical observations with uncertain hypotheses. A recent study by Chauviré et al. (2023) provides a robust scientific framework for analyzing and explaining opal degradation mechanisms.
Photo : Unstable chocolate opal nodule from Mezezo, Ethiopia © Malux Minerals
Understanding the fragility of opal
Opal is a hydrated form of amorphous silica (SiO2·nH2O), with a water content that can vary from 3 to over 10% by mass. This water is present in free molecular form (H2O) and in silanol groups (Si-OH). It plays an essential role in the mechanical stability of the stone.
The water contained in opal can be distributed in different types of porosity : closed, interconnected, or open. This configuration determines how the opal reacts to changes in temperature, humidity, and pressure.
Two crazing mechanisms identified
1. Drying shrinkage
This mechanism primarily affects freshly mined opals, rich in water, and stored underwater. When exposed to ambient air, rapid drying leads to volume contraction and the development of internal stresses. These stresses sometimes exceed the stone's mechanical strength, generating a network of microcracks.
Experiments show that this type of crazing can occur at room temperature. The stress is higher when the evaporation rate is rapid and the structure is porous. The persistence of residual mechanical stresses is confirmed by Raman analyses and observation under polarized light.
2. Thermal decrepitation
This phenomenon primarily affects opals that are already dry but still contain water trapped in closed pores. When heated to high temperatures (>250°C), this water boils, creating sufficient internal pressure to cause sudden fracture. This process is similar to that observed in fluid inclusions of quartz or corundum.
Affected opals do not show residual stress after crazing, but their brittleness remains due to the presence of water encapsulated in sealed microcavities.
Factors influencing stability
The stability of an opal depends on several interconnected parameters :
- Total water content : varies depending on the origin and structure
- Type of porosity : closed pore = thermal risk ; open pore = risk in open air
- Drying rate : slow drying limits capillary stress
- Storage conditions : exposure to dry air or heating can cause cracks.
So-called "hydrophane" opals, which are very porous, have a significant water absorption capacity but, paradoxically, are more stable when dried slowly.
Prevention and best practices
Opals, particularly those with high water content, are sensitive to environmental variations that can cause gradual cracking or even complete disintegration. To limit these risks, it is essential to adopt preventive measures adapted to the unstable nature of certain specimens.
- Natural air drying : After extraction, opals must be allowed to dry slowly at room temperature and away from any direct heat source. This phase, which can last several months, allows for the gradual evaporation of the water contained in the amorphous structure of the silica, thus reducing the internal stresses that can cause cracks. Premature release to the market significantly increases the risk of degradation.
- Avoid sudden changes in temperature and humidity : Opals must be protected from unstable environments, such as display cases lit by halogen lamps or areas subject to dry air currents. A stable and moderate relative humidity is preferable. Storage in sealed containers, possibly with a regulated humidity source (such as pre-conditioned silica gel), can help maintain moisture balance.
- Temporary storage under water : For recently mined opals or those showing signs of brittleness, temporary storage under water can slow moisture loss and prevent internal stress. However, this method should remain temporary. Prolonged storage without prior investigation may mask instabilities or promote slow chemical changes, such as the growth of microorganisms or the leaching of soluble elements. Any opal sold in sealed bottles or in water-filled containers should be considered highly likely unstable.
Photo : Unstable Ethiopian opal preserved in a water-filled bottle © Mineral Mike
Stabilization methods under study
Given the vulnerability of certain opals, particularly those from Australia and Ethiopia, more advanced techniques are currently being explored in laboratories or within the framework of supervised artisanal practices :
- Supercritical drying : This method, borrowed from the porous materials industry, removes the water contained in the stone without undergoing a liquid-to-gas transition phase. By avoiding the capillary forces responsible for internal contractions, supercritical drying significantly limits the risk of cracking.
- Impregnation treatment : This involves introducing substances such as mineral oils, polymer resins, or colloidal silica gels into the pores of the opal to fill the voids left by water evaporation. This technique improves the cohesion of the material and strengthens its mechanical strength, but it sometimes alters the visual appearance (brilliance, color) and must be clearly communicated to the collector or buyer.
- Non-destructive diagnostics : Before considering any intervention, it is recommended to carry out a stability diagnosis. Methods such as near-infrared spectroscopy (NIR), differential scanning calorimetry (DSC), or density and porosity measurement make it possible to assess the hydration rate, internal structure and the probability of crazing in the medium term, without damaging the sample.
Towards a predictive assessment of stability
The challenge ahead is to develop reliable tools to predict whether a given opal is stable or prone to crazing. Mechanical properties, which are poorly documented, should be better explored. A correlation between density, permeability, pore size, and fracture toughness could provide a solid basis for a gemological sorting method.
In the meantime, prolonged observation of stones in the open air remains an empirical but effective method. If the opal shows no alteration after several months, it is generally considered stable for use in jewelry.
Conclusion
Thanks to recent research, opal cracking no longer appears to be a random or mysterious phenomenon. It results from identifiable physical mechanisms : decrepitation for dry stones, and shrinkage through desiccation for those that are still wet. A better understanding of these processes paves the way for safer practices for extracting, storing, and marketing this precious gem. Ultimately, this scientific approach could restore confidence in certain natural opals and enhance the value of deposits previously overlooked due to their apparent instability.
References :
CHAUVIRÉ, B., MOLLÉ, V., GUICHARD, F., RONDEAU, B., THOMAS, P. S. & FRITSCH, E. (2023). Cracking of gem opals. Minerals, 12(1), 90.
FILIN, S. V. & PUZYNIN, A. I. (2009). Prevention of cracking in Ethiopian opal. Australian Gemmologist, 23(12), 579–582.
GAUTHIER, J.-P., MAZZERO, F., MANDABA, Y. & FRITSCH, E. (2004). L’opale d’Éthiopie : Gemmologie ordinaire et caractéristiques exceptionnelles [Opal from Ethiopia: Usual gemology and unusual characteristics]. Revue de Gemmologie A.F.G., 149, 15–23.
JOHNSON, M. L., KAMMERLING, R. C., DEGHIONNO, D. G. & KOIVULA, J. I. (1996). Opal from Shewa Province, Ethiopia. Gems & Gemology, 32(2), 112–120.
KOIVULA, J. I., KAMMERLING, R. C. & FRITSCH, E. (Eds.) (1994). Gem News: Opal from Ethiopia. Gems & Gemology, 30(1), 52–53.
RONDEAU, B., FRITSCH, E., MAZZERO, F., GAUTHIER, J.-P., CENKI-TOK, B., BEKELE, E. & GAILLOU, E. (2010). Play-of-color opal from Wegeltena, Wollo Province, Ethiopia. Gems & Gemology, 46(2), 90–105.