A Chalky; Chalkie (or "Chalkies"), is a nickname given in skateboarding to small rocks or other small items that can wedge under the wheels. Chalkies pose an immediate threat of suddenly stopping the wheels movement and throwing the skater off.
After a stone is wedged it moves several centimeters before stopping the wheel all together. It tends to leaves behind a line similar to a chalk line, marking the position of where you were thrown. Hence the name, Chalky.
-"Oh man, how did you stack that bad?"
-"I was skating and hit a chalky, immediate faceplant"
-"Some chalkies coming up ahead, best to avoid them."