I've heard that working horses on hard surfaces in general strengthens horses bones. But I think the key is building up the time and distance very gradually so you don't over stress them. I do know that the barefoot trim people recommend working horses on gravel to toughen the feet, but that's about all I've heard on gravel specifically.
Hmmmm....I’ve thought the key to strong bones, was of course working them over time, and proper feed. I’d never heard that working on hard surface makes a difference other then toughening feet.
Becky