One of the most interesting locations for precious opal is situated in Nevada: the Virgin Valley. Many areas are filed and claimed for opal-digging. Approximately 14 millions years ago this area was buried with volcanic ashes. The included woodlogs decayed within the time. The free cavities were then filled up by opal. Today the digger can find opalized limbcasts (without preservation of the wood structure) and in addition, partial opalized wood.
Many color and structural variants of the precious opal are found in the Virgin Valley, among others white opal, crystal opal, black opal and relatively frequently Contra-Luz opal. This type is showing the opalizing-effect only with penetrating light.
Despite their beauty many of the opals from the Virgin Valley have a big problem: They must be kept in the water - otherwise a majority of them is getting cracks and fissures.
Several miner are permitting collectors to search in their rough material (www.virginvalleyopal.com; www.royalpeacock.com; www.nevadaopal.com…). The fees are very different - depending on the type of the provided material.
Scott Kleine posted an interesting article about Virgin Valley opals in the internet. Also Peter C. Huber describes the find situation in the Virgin Valley.