What is the irony in the story about the miner and the emerald?

The old man comes to Santiago because it is the old man's destiny to help guide people who are trying to realize their destinies. The old man tells the story of the miner who believed it was his destiny to find an emerald. He searched and searched for five years for his emerald in a riverbed. To help him achieve his destiny, the old man disguises himself as a stone:

Since the miner had sacrificed everything to his destiny, the old man decided to become involved. He transformed himself into a stone that rolled up to the miner's foot. The miner, with all the anger and frustration of his five fruitless years, picked up the stone and threw it aside. But he had thrown it with such force that it broke the stone it fell upon, and there, embedded in the broken stone, was the most beautiful emerald in the world.

Situational irony is when something happens that is opposite of what we expect. The old man expected the miner to find his emerald and thus to achieve his destiny, but just as the miner had it in his hand, he threw it away.
The novel moves on, so we don't know if the miner saw the emerald as it broke open to reveal itself, but the irony is that he was ready to give up in anger just as he was on the point of fulfillment of his dream.