It’s amazing how the prettiest rocks show up where you’d least expect them …
Nestled in the Sierra Nevada foothills due east of Manteca, CA, the BLM’s Red Hills Recreational Management Area is usually bypassed by busy tourists and weekend campers, headed for the scenic vistas of Yosemite and other Gold Country recreation areas. Happily, more and more rockhounds are discovering the unexpectedly colorful dendritic opal that lays hidden within the BLM Red Hills ACEC. The opal occurs in seams in these highly eroded serpentine hills, and can also be surface collected over a wide area surrounding the seams.
Be forewarned, the opal in these hills shows no precious ‘fire’ or play of color. That noted, with patience in digging and seasoning, lapidaries can cut stunning opal cabochons in a wide variety of colors, accented by dendritic inclusions of manganese oxide.
Being a bit of a slow poke, I’ve only covered a small portion of these red hills. The surface ground is generally quite loose, due to the high iron oxide content of the serpentine. The Red Hills are, quite literally, a pile of rust.
Surface collecting is quite easy; look for the generally small white to peach stones, which look much like broken bits of porcelain plates. Surface collected opal can generally be cut immediately, having been exposed over time, and fully dried. If you choose to dig in one of the exposed opal seams, you’ll find a much larger and more colorful variety of dendritic opal. The answer to why almost none of the more colorful material has survived surface exposure is simple; it contains too much water.
Opal, when ready for cutting, has an ideal water content of 6-8% of the total volume. Chinese Camp dendritic opal, when freshly unearthed , has an average of 15-20% water in its makeup.
Yes, I know that it looks so lovely when you’ve just brought it into the light, but you need to immediately put the best smaller chunks into your bucket, without washing them too thoroughly. This will allow the opal to begin the slow process of dehydration that will assure maximum material integrity. Larger chunks should be left unwashed, and secreted away in a cool, dark place, like a garage.
After about 3 months, it should be okay to take some out and clean it up. Try sawing a chunk or two, to see if it fractures or crumbles away. I generally wait a year before trying out the opal, and have some that’s been waiting 3 years to be cut. It’s quite a bit harder now!
Be sure to use a water cooled saw, with clean coolant, to avoid discoloring any of the magnesite that might be included in the opal. Any porousness will pick up and retain any colors or odors in an oil lubricated saw. If you feel the need to seal any holes or porous areas, a simple SuperGlue sealing will do the trick. I’ll be happy to share instructions, upon request. You’ll also find a link to a pdf file with the instructions, here.
Directions - Google Maps link for Chinese Camp sites.
From Manteca, CA - Take Hwy 120 East from Hwy 99, through Oakdale, and turn right at the Yosemite Junction, where Hwy 120 turns southward. In a few miles, you’ll see the Industrial Park to your right. Take a right through the gateway, onto Enterprise Drive.
Take Enterprise Drive west, and turn left onto High Tech Drive. Cross the railroad tracks, and turn right onto Serpentine Loop Road. In a few tenths of a mile, you’ll see the parking area, just before the railroad tracks go into a road cut. You’ll find the diggings and collecting areas to the north of the RR tracks, about 100 yards down the cut. If you find old, deep diggings, keep walking in. You’ll find it!
From Mariposa, CA - Take Hwy 49 north until you PASS Chinese Camp and turn onto Hwy 120. The turnoff for Enterprise Drive will be on your left in less than 1.5 miles.
Remember, camping isn’t allowed in the Red Hills ACEC. However, you’ll be close to the best camping areas of the Gold County. Here's a Yelp! link.
Enjoy your day in the Red Hills!