Cheapest priced copy (The Brothers' War): $0.35 at Card Kingdom, near mint, their price list as of .
Horned Stoneseeker 
Creature — Lizard · 2/2 · Uncommon · The Brothers' War · #138
Menace
When this creature enters, create a tapped Powerstone token. (It's an artifact with "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell.")
When this creature leaves the battlefield, sacrifice a Powerstone.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Brothers' War | 138 | Uncommon | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 14 October 2022Powerstone tokens are a kind of predefined token. Each one has the artifact subtype "Powerstone" and the ability "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell."
- 14 October 2022Although all the cards in The Brothers' War that create Powerstone tokens create a tapped Powerstone token, entering the battlefield tapped isn't part of the token's definition. Notably, if you create a token that is a copy of a Powerstone token, the token copy won't enter the battlefield tapped.
- 14 October 2022You can use the {C} added by a Powerstone token on anything that isn't a nonartifact spell. This includes paying costs to activate abilities of both artifact and nonartifact permanents, paying ward costs, and so on.