Let thieves snatch at least 1 million;

Thieves only appear on the Relic stages and don't move under their own power. They move just like you do; by sliding around the stage. Because there are no idols on these stages, gems really don't matter so just focus on getting the Relic and let the Thieves get anything else that is in their path. Just like your stash, the amount they amass is cumulative over all the Relic stages. This unlocked for me about midway through the game.