Please tell us where you found it. It could be sepentine. A riverbed tells us only that but a riverbed in northern Utah tells us more. You say it very hard but not how you determined it was hard. You can scratch it with a piece of quartz which is known to 6.5-7.0 on the Mohs scale, or try to scratch it with a common nail. If you cannot scratch it, you will know it is very hard. You could also do a streak test with a ceramoc tile. The color of the streak will tell us more, too.