Wiktionary
n. Stone formation made up of calcite and other minerals that build up in cave pools.
Wikipedia
Rimstone, also called gours, is a type of speleothem (cave formation) in the form of a stone dam. Rimstone is made up of calcite and other minerals that build up in cave pools. The formation created, which looks like stairs, often extends into flowstone above or below the original rimstone. Often, rimstone is covered with small, micro-gours on horizontal surfaces. Rimstone basins may form terraces that extend over hundreds of feet, with single basins known up to 200 feet long from Tham Xe Biang Fai in Laos
Usage examples of "rimstone".
The light from his headlamp caught a rimstone pool in the middle of the chamber.
Having crossed the rimstone pool to wander at the other end of the chamber, he stood absolutely still, noting the acidic moldy odor of bat shit.
At her feet lay curve after curve of multilevel rimstone pools, a fantastic lotus of silver and gold and jade unfolding, revealing the woman seated amid a beauty that could be equaled only by the singing of her dreams.
And on the far shore a creature that raised its dripping mouth from the rimstone pool and stared into the light with eyes dead white and sightless as the eggs of spiders.