It is often difficult to find a good source for fireplace hearths. There are allot of firms retailing various sizes and the entire process can get confusing. Large firms such as B&Q stock light weight fireplace hearths, which is a thin sheet of marble supported by a metal frame. Specialists (such as Creative Fireplaces) would always supply hearths made out of solid sheets of marble or granite. Essentially a fireplace hearth can be a flat piece of marble, however it is conventional to have s vertical section to create a false sense of depth (or make the hearth look deeper than it actually is).  A specialist hearth filled with cement (or better known as a Slabbed hearth) is needed if you will be using solid fuel (E.G Burning coals or logs). The expansion joints in the hearth and the cement backing will ensure that the hearth is more dense and minimise the chances of it cracking during the expansion and contraction stage.