A quick subjective retouch; I think these "gems" could be exactly like the gem of true sight(without giving the true sight of course :D ) you take it, drop it back to your base, and an enemy can pick it up from your base. Or if you die, the gem is dropped and can be picked by anybody. We can limit that all heroes can only…
If we were going to do a lava themed map we should use the standard maps floor as it just doesn't really fit. We would have to redesign it and a great example of the would have to be black rock mountain from hearthstone/world of Warcraft