This is an ancient thread, but I thought I would add something for anyone who might stumble in here, looking for info about diamonds in Oregon, Washington, California, or Idaho.
After a bit of research, my very strong suspicion is that there are *a lot* more diamonds in Oregon than is suggested by the fairly small number of recorded (and verified) chance discoveries by gold placer miners (although hundreds of not-tiny diamonds have been found in California).
Thing is, darn close to no one looks for diamonds along the Pacific Coast. The Old Timers working the gravels of the Klamath and Sierra Nevada Mountains generally didn't know what they were. Heck, they look like an odd little piece of quartz or just a weathered bit of agate, if you just glance at them. And sluices filled up with black sands will bounce diamonds out.
I have actually, recently, conversed with an older gentleman who has been a professional prospector for years, who told me he was present when a mining partner panned 2 little (but no micro) diamonds out if their sluice tailings.
In this part N. America -- well away from the ancient cratonic lithosphere, where large commercial operations are usually located -- diamonds seem to be associated not with any old volcanic rock, but with big sections of oceanic crust and upper-mantle that got shoved up (obducted) onto the North American plate These rock sections, called "ophiolites," are largely made up of varieties of peridotite (or serpentinized peridotite) -- which is mantle rock.
So the diamonds -- probably *very sparsely* disseminated, compared to the diamond in commercially viable diamondiferous kimberlite pipes -- might be eroding right out of big, extensive bands of exposed ophiolites. But the other possibility is that relatively shallow diamonds (compared to diamonds under thick cratonic crust) are being sampled at depth by magma and brought towards the surface in intrusions, and eventually weather out of some type or another of mafic or ultramafic dikes or sills. Or both could be true -- hosted in ophiolites and intrusions. Or I guess there could even be some (probably lamprophyre) diatremes (pipes or maars) that we haven't found, so *extrusions*.
Taking the Klamath Mountains for example, you've got ophiolites all over the range from when terranes (volcanic islands or whatever) welded to North America. And you have lamprophyre-type intrusions, that could have sampled diamond out of a slab of subducting plate that got stuck in a zone with the right pressure and temp for diamonds to remain stable -- or something along those line. Subduction zone diamonds.
Ad far as I can tell, this is a not really well understood by geologists. And economic geologists don't know for sure what the host rock source is of Pacific Coast region placer macro-diamonds -- particularly in the Klamaths and Sierras.
Since I don't know the Sierra Nevadas as well, if I were going to go look for diamonds, I would look for rivers in the Klamaths that cut through a lot of miles of ophiolites -- rivers with big gravel bars. Even if the diamonds are very sparse in the host rock, quite a lot of them could have accumulated over long periods. (Oh, and IIRC, diamonds have been found particularly in placers fairly rich in platinum group metals and chromite). Research diamond placer mining. Off the too of my head, I might check out some of those huge gravel bars in the S. Umpqua -- but there are many rivers in N. California and SW Oregon that would seem promising.
Some beach combing might be in order too, near the mouth of the Rogue and Chetco, and such. Keep doing that until you have several hundred little bits of "agate," then carefully examine each one of them.
Most of California, Oregon and Washington are made up of accreted terranes. There are exposed ophiolites in the Blue Mountains, and some in Washington, and a lot in the Sierra Nevadas in California. And in B.C., and Alaska. And, indeed, Western Idaho. Break out the geological maps.
I don't reckon we are going to know how many there are until people actually start looking for them.
