You may use flagstone (sedimentary rock) when constructing a fireplace hearth. Be very specific when buying flagstone however. Besides being a specific type of sedimentary rock, flagstone may also be used as a term used to describe stratified stone that can be split into level pieces for paving, or construction of various structures. You can have quartz flagstone, or marble flagstone for instance. In those cases the term means "flat stones" or "pavers" rather than stone type. The questions to ask your stone yard agent are, "What kind of flagstone is this?" and, "Is this suitable for use as a fireplace hearth?"