Sindūra (ལི་ཁྲི།, Wyl. li khri , sindoor in Hindustani) or vermillion is a pigment made by grinding a powder of cinnabar, the ore that contains mercury. how used, what symbolizes. The best kind is found underwater in zones not exposed to sunlight. It can be found in caves. This red pigment is one of the substances used in tantric rituals. Red is the colour of magnetizing.
Cinabar can be found on all continents, and in particular it is said to be present on all the sacred places such as 24 sacred places, or the 32 hallowed lands where the Tantric teachings are flourishing, whereas in the places where Dzogchen is flourishing one would find crystal. Sindura extracted from these sacred places is said to be especially blessed.
Chögyam Trungpa, The Heart of the Buddha, Page p.190. Robert Beer, The Handbook of Tibetan Buddhist Symbols, p.24-25. T.&M. Blau, Buddhist Symbols, p.157.