Floating Rocks

Floating rocks (synonyms: hovering stones, sky islands; example toponyms: The Hardclouds, the Floating Isles) are locations where boulders or large island-like rock formations float above the ground. This gravitational curiosity may be due to a strange property of the rocks, may be due to odd fluctuations in the local gravity, or may be magical in nature. The floating rocks may represent a local hazardous area, or the entire world may be this way, with each rock being an area on which people may live and work. They may contain floating cities and airships may offer passage between airship ports. There may be bridge systems or vines that connect nearby adjacent rocks. There may be land beneath, or there may be nothing. These rocks may contain unusual minerals or material.
When designing floating rock fields first consider what forces keep them afloat. Are there any tides which shift their location, or do they alter their position in vertical space? Can these flying rocks be piloted using sails, engines, or other means?


What event could stop a rock from floating?