Is it just the black ones or the white marble as well?

just the black ones. i couldn't get to the lower level of any tiles in the crates but, from the top levels the white ones were fine it was just the black that were like this. not all black are this way, but many were.

The black marble on my fireplaces is rather fissured on the surface. Of course the mantle pieces are 170+ years old.

I would call the company and maybe email a picture.

If installed on a solid substrate I would imagine that they would not crack more because they would be firmly attached across the entire back surface, + the fissures would be filled in with grout. Actually for the type of house you are building I don't think the cracks in this would bother me at all cosmetically, as long as I could be assured of its stability.

i sent a photo to the company. they've been fantastic to deal with thus far (Paris Ceramics). They fill these fissures with grout?!

If a proper installation is done, it should be fine. Tile that thick should not be installed with thinset. It needs a good old fashioned mud bed. And that means it will be a lot thicker installation than you have perhaps accounted for with the rest of your flooring and will need some type of transition. Finding a tilesetter than has experience doing a mud bed may be problematical as well.

Our tile guy put down a mud bed that has some sort of metal mesh in it. Is that what we need? This guy seems very skilled so I've not questioned him thus far.

Are you on a slab? If not, did the builder take into account that you would be laying thick marble tile when the house was built? Just making sure the deflection of the floor was taken into consideration. A good tile guy should have calculated it before laying the tile, but I just thought I would mention it. You don't want to end up with cracked tile down the road.