Granite is a type of igneous rock
It is formed deep beneath the earth from cooling and crystalized magma. Then exposed on the surface by uplift and erosion. It comes in several colors including light grey to pink.
The Dragon's Lair in Daventry was blocked off by a huge chunk of granite forming a boulder. The town of daventry has water troughs made of granite outside its stables.
A haystack boulder, a glacial erratic (boulder) from the pink granite outcrop exists in Daventry.
The Isle of the Sacred Mountain was formed of light granite, and the Isle of the Beast is dotted with rough granite rocks. The dungeons of the Castle of the Crown are also built from gray granite stones.
The Ancient One ceremonial symbol for D represents granite as its material meaning.