Horizon Stone 
Artifact
If you would lose unspent mana, that mana becomes colorless instead.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL
Artifact
If you would lose unspent mana, that mana becomes colorless instead.