Nearly 14 million years ago, Virgin Valley, in the northwest corner of Humboldt County, was covered by lakes and forests. Today, what we see is simply bare hills. The entire area was several thousand feet lower and more like the coastal mountain ranges of today. However, the earth was not quiet; volcanoes periodically erupted, blasting the forests apart and burying them under hundreds of feet of ash. This cycle was repeated several times over the next million years. Magma later pushed to the surface and repeatedly flowed over the region. The layers of ash and blasted trees were buried more than 1500 feet deep.
Under millions of tons of rock and ash, one of the miracles of nature was taking place, transforming common silica into Fire Opal. The pieces of the buried forests were disintegrating and the ash surrounding them was being chemically altered by hot fluids into clay. From deep underground, super-heated water flowed upward through the cracks and faults in the silicon rich ash layer.
Current thought on how opals are created is that as the super-heated water moved through the ash layers, it dissolved some of the silica from the ash. When the water encountered a cavity created by the disintegrating wood, the water slowed, allowing some of the dissolved silica to be deposited.
Over the centuries, this process was repeated again and again, forming opals in the cavities left by the decayed wood. Above the ground, the climate and landforms slowly changed into what we see today - a high, arid desert, cut by rugged mountain ranges. But the opals have not changed.