Speed Leveling Legendary Gems

Gem Upgrade Chance / Greater Rift Level Compared to Gem
----------------------------------------------------------

    0% / -7
    1% / -6
    2% / -5
    4% / -4
    8% / -3
    15% / -2
    30% / -1
    60% / +0
    60% / +1
    60% / +2
    60% / +3
    60% / +4
    60% / +5
    60% / +6
    70% / +7
    80% / +8
    90% / +9
    100% / +10
 The most efficient way to get a level 25 Legendary Gem would be farming "9" Greater Rift Trial Keys, which drop off normal rift guardians/bosses. Than proceeded to complete Greater Rifts Level: 13, 16, 19, 22, 25, 28, 31, 34, 37; Which allows a 100% chance to upgrade your gem to tier 25 | PS. don't forget to hit upgrade gem every time you complete a greater rift