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A Useful Technique to Confirm the Hydrophane Nature of Opal

Richard T. Liddicoat Gemological Library

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Hydrophane Opal Treatment

Figure 1. Top: These five opal slices of comparable thickness were cut from the same rough. They were semitransparent to translucent when viewed in daylight on a black background. Play-of-color was displayed with oblique lighting at certain angles. Bottom: The slices’ transparency improved after immersion in distilled water for approximately 20–30 minutes. Each slice absorbed water differently depending on its porosity. Photos by Nuttapol Kitdee.

Recently in GIA’s Bangkok lab, an approximately 4 cm piece of rough hydrophane opal was fabricated into five slices for various treatment experiments. Each slice was approximately 0.5 cm thick. Only clean tap water was used in the cutting process, and no other materials besides opal were involved. The opal slices showed a whitish, cloudy appearance in daylight and appeared light brown to brown in calibrated diffused light.

After the cutting step, all five slices underwent specific gravity testing, which gave SG values of approximately 1.93 ±0.03. While immersed in water, the slices displayed obvious weight gain and improved transparency, which indicated that the material was hydrophane opal (figure 1).

The water-absorbing ability of the opal was found to vary through the whole piece, resulting from…