Slate is basically baked mud. The varieties that have been well baked, such as those from Vermont or Arvonia, VA are extremely non porous and never "need" to be sealed. Their appearance can be very nicely enhanced with appropriate "sealers" if desired.

There are multitudes of imported "slates" from India, Africa, South America, China, etc that vary anywhere from half baked hard mud to very dense quartziferious sandstones. Some are very durable, as davidmash's seem to be, and some are real junk.

The best DIY forum for advice on all things tile & slate, and other stuff, is the John Bridge forum. Lots of very friendly professionals willing to share their experience, and also lots of average Joe's discussing their mistakes & success stories. A very worthwhile place to sign in.

http://www.johnbridge.com/
or the forums
http://www.johnbridge.com/vbulletin/

These guys are the best, IMHO