A liquid-filled glass display dome with precious opals from Virgin Valley, Nevada, USA.  Opals from this locality form, and are mined from, an in-place deposit within a defined zone of bentonite/montmorillonite, which can be traced through the surrounding rock. Above, below, and outside of this mineralized zone, no other commercial opal mineralization exists.  A typical source deposit of precious opal.  A small percentage can be cut into gemstones, but most are kept as mineral specimens.  From the Wentzell Trust Collection.

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