Cheapest priced copy (Commander Legends): $1.79 at Card Kingdom, near mint, their price list as of .
Horizon Stone 
Artifact · Rare · Commander Legends · #315
If you would lose unspent mana, that mana becomes colorless instead.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $1.49 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends | 315 | Rare | $1.79 | $1.49 |
| Commander Legends | 694 | Rare | $4.99 | $6.99 |
2 rulings from Wizards of the Coast
- 10 November 2020If unspent mana you have has any restrictions or riders associated with it (for example, if it was produced by Jeweled Lotus), those restrictions or riders remain associated with that mana when it becomes colorless.
- 10 November 2020As long as Horizon Stone remains under your control, you'll retain unspent mana as steps and phases end, although that mana will become colorless. This means you can add mana and spend it during a future step, phase, or turn. Once Horizon Stone leaves your control, you'll have until the end of the current step or phase to spend the mana before it is lost.