Question
Is the wood suitable for burning in a fireplace as I know it is very hard?

Answer
Yes. Black Locust is unsurpassed as firewood for wood stoves; it burns slowly, with little visible flame or smoke, and has a higher heat content than any other wood that grows in the Eastern US, comparable to the heat content of anthracite".(Heating homes with Wood.) However, for this use it should be split when green, then dried for 2 to 3 years, and ignited by insertion into a stove already hot from burning of a load of some other hardwood. In fireplaces it is less satisfactory because knots and beetle damage in black locust make the wood prone to "spitting" coals for distances of up to several feet. If the Black Locust is cut, split, and cured while relatively young (within ten years) typically damage and "spitting" problems are minimal. It can be an excellent firewood in stoves, campfires, and fireplaces if properly cultivated.