The Only Rock That Floats In Water Is?
If you’re planning on floating an S.O.S. message off the island you’re stranded on and you’re short on bottles and coconuts, you could always attach it to a big hunk of pumice–the only rock that floats.
Pumice has an extremely low density, so low in fact that a given volume of it is 1/12th the weight of granite, 1/45th the weight of lead, and–most relevant–1/2th the weight of pure water. Drop a pumice stone in the ocean and it will make a splash like any stone, but then it will float away on the waves.