It might be archaeological find. A pilgrim badge from an old Dwarven religious shrine. Most of them were crushed down or repurposed after the shrine broke down. They're kind of a collector's item now, like old coins or baseball cards. A collector or museum might give you something for it.
Or it could be the Dwarven equivalent of firework poppers. Toss them on the ground and it gives off an interesting and harmless magical effect, like a tiny explosion, a small flash of light, or a ring similar to a bell.
Or it could potentially be a game piece. I'm imagining a game similar to Chinese Checkers or Go, except with square pegs instead of round. There are modern (read: contemporary to the game's time period) versions of this game that have more streamlined rules and are easier to play, but if you could find the matching gameboard, it could be an interesting way to burn an hour.
Or even it could be the keystone to a bridge which once stood in this area centuries ago. When the stone was removed, the bridge collapsed and was eventually buried by erosion (thus explaining the pile of gravel). These stones were special in that they are made to be resistant to the weather and are critical to the structural integrity of their bridges. Whoever took this out really wanted that bridge to not be there.