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.