In Susan Cooper's The Dark is Rising sequence, warestones are sometimes (once, really) used by the Light and Dark to moniter a specific place. (aware stones, 'warestones. Simple corruption, dropping of vowels.) The one in canon was a small, perfectly smooth quartz pebble. The Dark was keeping watch over a place and its inhabitants through the stone, but later on the Light was able to use the Dark's warestone for plot advancement and such.

I was walking to class today and saw a perfectly smooth white quartz pebble, and my first thought was of the warestones.