Most gem quality rubies and sapphires come from metamorphic rock with igneous intrusions, so you would be real unlikely to be able to get big clean crystals this way. On top of that, you can't really get them out of the metamorphic rock really because you'll break them with the surrounding rock. We mostly get them from sedimentary deposits (placer deposits) when the softer rock around them weathers away and the hard gemstones get picked up by water and moved downstream.