A rare combination: the classic look of the inclusion of a horse's tail in a clear stone.
Such stones are typical for the channel placer (aluvium, already worked out). Sometimes it is possible to hook a thin lens, which the "grandfathers" simply did not work out. It is too small for those times, 10-15 cm of thickness at a depth of 2-3 meters with a constant water inflow
Apparently, the famous term "horsetail" originated from such stones.
Origin - Bobrovskoye field, Tagil, Ural, Russia