wotc
You can't cast an instant (or activate an ability as an instant) while casting a spell. Therefore, much like Lion's Eye Diamond, you can't activate the ability intending to use the mana to cast a spell from your hand.
MTG · Card
◇ Catalog #470
Diamond Lion
mh2·#470·rare·extended_art
Illus. Howard Lyon
—
in vault
—
for trade
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wishlisted
Jun '21
5 yr ago
Value
$1.11+$0 (+0.0%)
30 days high $1.11 · low $0.19
Volatility · high
USD$0.19EUR€0.82
NM$1.11
Where to buy / sell
Search links · prices vary by venue
Buy this card
Played in EDH
EDHrec rank #10,777 · Fringe play
Variants
Gameplay
Artifact Creature — Cat
{T}, Discard your hand, Sacrifice this creature: Add three mana of any one color. Activate only as an instant.
2/2
Legalities
tlrlegal
duellegal
brawlnot legal
pennylegal
PreDH
Rulings · 2
wotc
Even though you can only activate this as an instant, it is still a mana ability. It doesn't use the stack and cannot be responded to.
Notes
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Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Howard Lyon
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.