Ratnapura alluvial gem gravel sorting: conical cane basket (Watti) swirling techniques?

Washing gem gravels uses conical woven bamboo or cane baskets (Gem Watti). Standing waist-deep in slow river currents at first light, miners load gravel and submerge the basket, swirling it in continuous circular pulses. The centrifugal action, combined with water buoyancy, washes light silica sand, mud, and light quartz over the basket rim. Denser mineral grains (corundum specific gravity ~4.0, chrysoberyl ~3.7, spinel ~3.6) settle straight down to the conical vertex at the bottom. The miner lifts the basket, flips it in sunlight, and picks gems concentrated in the central core.