We have a black marble floor that appears to have raised veins. In other words the white veins in the marble appear to be lifting up and in some places are falling apart. What causes this to happen?

I am assuming that the black marble you are referring to is negro marquina. If this is the case you will find that this type of black marble is very susceptible to spalling of the veins. Over-polishing with either powder polishes or recrystallization will cause this problem. The only way to fix this problem is to hone the floor and repolish.

TIP If the spalling is severe it may be necessary to hone the floor and then fill it with a black colored polyester. After filling rehone and then polish.