Bull quartz

A boulder consisting nearly all of quartz (SiO2, silica, silicon dioxide) at the old Crystal Peak Mine, in California very near the Nevada line. "Bull" quartz is an old miners' word for this sort of massive occurrence. It's pretty common--silicon (Si) and oxygen (O) are the most common elements in the crust--but it doesn't look like your run of the mill rock, being milky white to translucent and with a glassy or even waxy luster. So people tend to pick them up. I saw lots of specimens when students would bring in their "pet rocks" for a geologist to look at!
Sometimes (but by no means always!) gold occurs in bull quartz, and lots of the hard-rock mines in California's Mother Lode country were in gold-bearing quartz veins. So, when the 49ers found this massive occurrence, they really thought they'd hit a bonanza! But they'd hit borrasca instead; the quartz is barren. It was quarried commercially as decorative stone for a while, but has now reverted to public ownership. The US Forest Service, which owns the land, asks that people refrain from collecting more than one 5-gallon bucket per week! The inset shows smaller chunks just lying around in the old mine site. This is what people shovel into buckets.