(*Note: All photos are CC. As always, if I took the picture, feel free to re-purpose it to your own ends. Credit is nice but hardly necessary)
So of course I'm playing with my new digital microscope and you have to bear the burden of my findings. Below, a perfectly shaped minute quartz point. Sure looks like a diamond, but it probably isn't.
That said, if you looked at the previous pebbles I posted, there's a big grey area...lots of things you can say definitely aren't, but there's a few I'm unsure about.
For example, Ballas Diamonds. Harder than diamond, formed as a result of meteoric impacts, tiny, round, frosted spheres. Here's a selection of some I culled from the web:
Now look for similar in the photos I've taken. They're everywhere...and, if they were big enough, nice enough, might have some value in jewelry, rename them: "Carbon Pearls..." or some such, harder than diamond, they've got to have a value other than industrial.
And the so-called "Vitreous Luster", well, you'll see that everywhere...hardly an indication of anything...
The pictures, some are from different rivers, a big difference in the quality and type of stones found even over 10KM, in some dry creekbeds an abundance of perfectly formed garnets (not as good as BC, but we're a long way from the mountains...or the sample locations are...). 
Middle, left, the way it sparkles...
Lots of garnets here, and a few stones with that "Vitreous Luster".
Another good "sparkler" between the 2 bigger pebbles.
More stones, note the frosted possibly-Ballas stones. The spherical black one on the right middle side of the picture is a shotgun pellet that showed up in my pan.
Ballas?
Back-lighting washes out the color, looking for that "diamond" shape.
Perfectly shaped garnet, only slightly worn. Numerous "Ballas" and other suspects...
This is a sampling of three locations, hundreds, thousands to go...and the days are long and beautiful. Only work interferes...