What size granite or marble for fireplaces and bath vanity tops

I want to put crema marfil marble and absolute black granite on two fireplaces. Also want to use the crema marfil on several bathroom vanities. I can't get a straight answer on whether to use 2 cm or 3 cm marble and granite on fireplaces and bathroom vanities. Also most of the crema marfil is not good and has cracks and filler. Is this normal. Please help