Since outdoor tile and stone can become very slippery when it rains, I like to use a micro-etcher on any honed natural stone tiles, porcelain tile, or other potentially slippery paving outside. My favorite is a local company called Sliptech. Here is their website: http://sliptech.com/
Their product creates tiny pockets in the surface of the tile. We test it on each material before applying it on the job, but so far it has not drastically changed the finish or color of anything I have tested it on. I would probably be more hesitant to use it on a polished stone, since it may cut down on the shine, but I wouldn't typically use a polished stone outside anyhow.
Not having to worry about the slip factor opens up a whole new world of products for use outdoors. I especially recommend the treatment when doing a waterjet mosaic in the front entry, since these are often honed.