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.
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