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Cinder are light weight stones, (I don't know what exact material), which are used when we fill sunken parts under toilet floor.
In toilet slab, (floor actually) for piping purpose we have to provide that slab at lower level than the tilings, this remaining voids are filled with light wi\eight stones which are called cinders.

I am very new in the field of architecture, so I know only this much. But this is more-or-less true answer.