After only a few seconds of running away, Jake realized that the Zhorion he had just fought couldn't have been responsible for the heavy footsteps that had shaken the walls of the staircase. This was only possible with a heavy and massive creature. In other words, a dinosaur, or more logically another one of those humanoid monsters.
These creatures seemed devoid of any personality and they clearly obeyed a few privileged Zhorions to the letter. This warrior he had just killed was one of them.
That's why he was not so surprised when his path was obstructed by the hideous head of one of these monsters at the very first intersection.
The creature's head was more than a meter and a half in diameter with dark skin, yellow fangs, and pointed, partially jagged ears. The underpass was only two or three meters wide and high, and obviously had not been designed to provide easy access for such large monsters.
It is not mentioned, but Jake could indeed have tried to eat his own Green Stone. The reason he didn't was because it was the only stone he had.
As for eating metal or stones, Jake has tried it in small amounts, but it did not provide him with any tangible benefit over eating the same amount of normal food.
Edit for non priv: Second chapter of the day.