Figurative use: something as worthless as a broken tile or a common stone. Often used in self-deprecating or dismissive contexts.
Rare, possibly archaic or dialectal use for 'brick'. Modern Japanese normally uses 煉瓦 (れんが).
He speaks of his own work as if it were worthless, but it is actually wonderful.
In old documents, 瓦石 can refer to bricks.