This was a glitch that I experienced, and it may or may not happen for everyone.
I was playing on endurance and got a grade 13 3-part gem. My max mana pool was at about 200k, though with the “auto-expand” function on (holding shift hitting the mana pool spell) and with the game paused, smashing the grade 13 gem on my anvil put me at about 500k/200k mana. After waiting for about 10-15 seconds, I got the journey amulet without actually expanding my pool.
My guess for why this occured was that since my mana somehow tipped over the max, this told the game to auto-expand the pool. But since it was over the max and not actually at the max, and considering the game was paused, no mana was actually subtracted and the amulet was quickly awarded.