I bought a sack of 3/8" river rock
from a local rockery for about $16. Damn thing must have weighed
100 pounds! Compared to the other kinds of rocks, these were about
4 times the cost. Apparently pretty rounded rocks are
River rock is composed of various types of rocks that are generally
rounded, having been naturally tumbled in a river bed.
Rockeries near me sell what they call PAMI, river rock pebbles
from western Montana. They're also call rainbow river rocks. As
you can tell from the picture, there's a good reason for that name.
They're naturally and beautifully colorful, once they're rinsed.
I sorted them by hand; small batches at a time; culling the roundest
pebbles; discarding anything that was jagged, cracked, rough. Yes,
it is a mind numbingly tedious process, examining each pebble by
sight, touch and/or feel. But such was my determination.
If you know a rock hound ask for great nicely tumbled
(but not polished) rocks. That may save you from the mind numbing hand
The first round of culled pebbels looked
very sweet. The site of naturally gently rounded shapes of various
sizes and earth tone colors is soothing, in a way. :)
When I had more than enough to properly fill my vibrating tumbler,
I tossed in the pebbles and a batch of
unsanded lentil beads and turned the machine on. There was quite
a racket because rocks are banging against the plastic tub. The
the unique motion created by the vibrating tumbler is on track.
I slowly poured in a
little bit of water, noting that too much water will
suppress the action. The water immediately dampened the
noise. I added just enough water so that everything was wet.
Tumble time: 6 hours.
It worked! I kept the machine running while it slowly cycled the
beads to the top, where I pluck them out of the bowl. Then
I stop the tumbler and pour the rock 'soup' into a colander and
rinse the rocks, separately rinse the beads.
I'd estimate, because of the coarsest
rocks, the beads had a sanded smoothness equivalent to CAMI 400
- 500 grit.
I've called this set of rocks - stage 1.
I revisited my batch of river rocks, the rounded ones, and culled
out the smoothest ones from the not as smoothest. It wasn't too
hard to visually distinguish between especially when the rocks
are dry. This second culled batch, called stage 2 because of
the smoother stones, I expected would abrade at a finer grit
The smoother ones didn't look too different from the less smooth
ones, but they definitely felt different! :)
I filled my Lortone 3A three quarters full with a mixture of those
stage 2 stones, the tumbled lentils and enough water to barely
cover the top of the batch. I used my rotary tumbler simply because
I didn't have as many of the really smooth rocks. Tumble time:
It worked beautifully. I'd estimate the beads had a sanded smoothness
equivalent to CAMI 700-800 grit. They buffed beautifully.
I tried rotary tumbling with highly polished stones.
I used my rotary tumbler simply because highly polished stones tend
to be semi-precious and expensive. I only purchased a pound of these
stones. They didn't seem to work too well. Perhaps being so polished,
they've not got sufficient teeth to abrade anything.
Finally, after much research, a couple of helpful landscaping
store employees and several very knowledgable members of a local
gem and geology society, I found out what these rocks are considered.
Collectively, they bandied about terms like:
- pixie pebbles
- rainbow river rocks
- western Montana river rocks
- A metamorphic rock, intermediate between shale and
- A rock containing chiefly clay materials; derived from
claystone, siltstone, or shale; used locally as building
stone, although rarely produced commercially.
- A fine-grained formerly
sedimentary rock that morphed; composed predominantly
of indurated clay particles.
Argillites are basically lithified muds
and oozes. They contain variable amounts of silt-sized
particles. The argillites grade into shale when
the fissile layering
typical of shale is developed. Another name for poorly
lithified argillites is mudstone.
These rocks, although variable in composition, are typically
high in aluminium and silica with
variable alkali and alkaline
The term pelitic or pelite is often
applied to these sediments and
rocks. Metamorphism of
argillites produces slate, phyllite,
and pelitic schist.
That's what I get for asking! :D
OK, so the PAMI (a.k.a. 'pixie pebbles' and 'rainbow river rocks')
I got at the local rockery is a collection of several different
types of rock from northwestern Montana. The most noteworthy characteristic
are the color variations. Any random collection is simply beautiful,
hence the name "Rainbow River Rocks". I shall call mine "Rainbow
Sanding Stones" since I've put my colorful collection of rocks
to work sanding my polymer clay pieces.
The predominant river rock from this region is, argillite, a clay
rock. Thus I'm using natural clay to sand polymer clay. For some
reason, this seems elegantly appropriate... or amusing... or both.