We recently had a concrete hearth made and installed in our new home. The colour was charcoal and the colour was very consistent throughout. After additional grouting work was completed on the fireplace above the hearth, we noticed that the hearth looked mottled and there were small dark spots on the sides of the hearth. Our builder told us that this happens due to the natural curing process of concrete. Is this in fact the case, or is the mottled look a result of messy tile and grouting work?