Check your marble!
It's perfectly acceptable to have a replacement marble on an antique, but make sure it is only the marble that is new and the iece of furniture it is sitting on is genuine. Check the under side of the marble to see if there is a grease stain from years of people running their hands around the rim. The darker the stain, the older the piece of marble in all likelihood. Old marbles can sometimes be put onto new pieces though so be careful!
Does it match?
There's a saying in the antique trade that goes like this. 'If a piece of furniture will come to pieces, in time the piece will end up in pieces in different places.' Many pieces lost their parts in the 1st and 2nd world wars. Wealthy families died or went bankrupt and the pieces went into smaller houses with lower ceilings. During this time, the pieces that could not fit into the home were burnt as firewood. So ensure that the parts match. I. e. The top of a bookcase matches the bottom. If it does not, this is called a marriage and you would expect to pay considerably less for a marriage.
This is a gorgeous piece of marble that I found in France recently - I just love it!