The first time was a coincidence. Amidst the murky waters of the dimly lit cave, the goblin found a shining stone. He wasn't the brightest fellow, and had no clue about the magic jewel's value or power. But he did know that it was very beautiful, and that was the most important thing to him. "I want more..." And so began his days of jewel hunting.
During Normal quest, in Chapters 70-3, 71-3, 72-1.
A goblin is a legendary evil or mischievous grotesque dwarf-like creature. They are attributed with various (sometimes conflicting) abilities, temperaments and appearances depending on the story and country of origin. In some cases, goblins have been classified as constantly annoying little creatures somewhat related to the brownie and gnome. They are usually depicted as small, sometimes only a few inches tall, sometimes the size of a dwarf. They also often are said to possess various magical abilities. They are also very greedy and love gold.
Artwork by Bjorn Hurri from OpusArtz.