Ratnapura alluvial ruby mining: distinguishing true corundum from red almandine garnet in gravels?

Almandine garnet and red corundum (ruby) occur together in Ratnapura alluvial gravels (Illam). In the field, miners distinguish them using tactile and optical screening: Garnet is isotropic (singly refractive) and appears dark or blackish-red with rounded dodecahedral water-worn pebble shapes and a lower specific gravity (~3.9-4.1). Ruby is anisotropic (doubly refractive), possesses hexagonal crystal geometry with basal striations, and exhibits higher density (~4.0) with strong dichroism (purplish-red to orangey-red) visible under a handheld calcite dichroscope.