I just finished to lay out natural stone tiles and was about to put on a chemical sealer I brought along with the stones (see this French product site, sorry, I can't find an english page), when I found a site where people where discussing ecologic alternatives (Futura sciences).
Their conclusions and tests have shown that a "hard-oil", which is normally used for wooden parquet (90% fir tree oil, 10% thinner), has given the best sealing effect, but it changed (darkened) the original color. The other alternative was Savon noir (Black soap). Lesser sealing effect, but preserves color better.
What is your experience with natural sealers for stone floors? Ideally I'd use an ecologic one, but if the chemical has much better performance and preserves my floor better (thus gives them a longer life) I will rethink.