Forum Thread
Price Check
Forum-Index → Discussion → Price CheckIf you go to the Gem Collector and check out his eggs he'll tell you how many of each Gem you'll need for one of his special eggs. When you get 75 Dragon Gems plus x amount of other gems he'll give you an egg of your chosen Legend in exchange. He can only give you Xerneas, Yveltal and Zygarde though.
EDIT: The two above explained it as well but I hope I gave more detail~
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