they've been using marble around fireplaces for hundreds of years. it's
actually easier to do than tile, since it's so soft. it does stain though,
so you have to be careful with the smoke and glasses of red wine on the
hearth.

whoever told you that you can easily replace tiles to change the color
shouldn't be listened to.

thinset is thinset. add your own admix, it comes in large jugs. there is no
porcelain or ceramic thinsets. porcelain IS ceramic, it's just fired in a
hotter kiln than lowfired ceramics to make it. the thickness of the tile
doesn't have anything to do with ceramic as opposed to porcelain; it's just
the style and/or type of tiles that gives a difference. some ceramic tiles
are very thin (mostly wall tiles).

I thought marble was hard, harder than man made tile. Maybe I'm
thinking of granet but I thought marble was hard and breaks easy when
cutting. Thanks for you ideas.