An Ilkorin-only root in The Etymologies with derivatives having to do with pebbles (Ety/BIRÍT). Its most notable derivative is the river-name Ilk. Brithon “Pebbly”. This name continued to appear in later versions of The Silmarillion, and given its location in Western Beleriand presumably it (and related words like brith “gravel”) became Sindarin.
Reference ✧ Ety/BIRÍT ✧ BIRÍT