Noticeable seams?

I have just had a granite look-a-like worktop installed
by a friend and his company and the seams show
quite badly. The walls have now been tiled and
although I threw a wobbler and he said he would
have to get someone else to re-do the stress and
inconvenience I have had with their various bodges
is more than I can stand to be repeated. It is
a dark mottled worktop. I dont think the seams
will let in water but they should be unoticeable
as they are in some kitchens. His workmen claim
I chose a difficult worktop to join but that's
rubbish because I have seen the exact same
one where it is almost impossible to see the seams.
Is this a common problem and can anything be done?