1615. Molding
The idea to make it so that the mana gems would only hold natural mana was not as simple as it sounded. Walker was able to store natural mana within the eternal orb because it had been made to house every single elemental mana which included natural mana when they fused together.
The normal mana gems usually could only host one singular mana. That meant that it would most likely be unable to hold all the elemental manas to combine in to natural mana. In turn, that would make them unable to store mana without Walker adding the natural mana directly. Unless he somehow managed to inscribe a natural mana rune on to them, which he did not have the ability to do because natural mana runes were not something he knew.