Natural stone tiles are distinct and unique

You should always view several pieces of the tile that is to be installed before the installation begins.  Since it has been created by nature, not only are no pieces exactly alike, those two pieces may not even be very similar.
The most common natural stone tiles include

Slate, flagstone (sandstone), marble, granite, travertine and limestone.  Each stone has unique characteristics and maintenance requirements.
Generally, you should seal natural stone tile before grouting

Unless you are planning on using the grout color in the stone as a design element.
Most natural stones are not resistant against common household acid (lemon juice) or oil stains

Therefore, you should use a penetrating sealer for all natural stones after installation.  Follow manufacturers instructions on frequency of re-application.

The disadvantages of natural stone

    Ongoing re-sealing program is required
    Limited chemical and stain resistance
    Higher installation costs than ceramic tile
    Product received seldom looks like the sample

    Timeless, unique appearance
    Design capabilities of through-body color material
    Perceived value
    Durable countertops and floors (granite)
    May be re-polished if scratched

The disadvantages of natural stone

    Ongoing re-sealing program is required
    Limited chemical and stain resistance
    Higher installation costs than ceramic tile
    Product received seldom looks like the sample