Memory Crystal 
Artifact
$2.23 #24089 0.19 28.3 years
Buyback costs cost 
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Cards similar to Memory Crystal expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Stone Calendar89%
$14.91↑#205420.37↑32.2 years↑
"Spells you cast cost 
less to cast."
less to cast."
Helm of Awakening86%
$1.61↓#1927↑0.32↑29.7 years↑
"Spells cost less to cast."
less to cast."
Spellwild Ouphe84%
$0.29↓#16690↑0.09↓19.5 years↓
"Spells that target this creature cost less to cast."
less to cast."
Emerald Medallion82%
$5.16↑#582↑0.30↑29.0 years
"Green spells you cast cost less to cast."
less to cast."
Pearl Medallion82%
$7.43↑#429↑0.1929.0 years
"White spells you cast cost less to cast."
less to cast."
Sapphire Medallion81%
$7.87↑#354↑0.34↑29.0 years
"Blue spells you cast cost less to cast."
less to cast."
Naiad of Hidden Coves80%
$0.22↓#10618↑0.06↓6.7 years↓
"During turns other than yours, spells you cast cost less to cast."
less to cast."
Hardened Berserker80%
$0.04↓#243520.00↓11.6 years↓
"Whenever this creature attacks, the next spell you cast this turn costs less to cast."
less to cast."
Geyser Drake80%
$0.13↓#8697↑0.192.5 years↓
"During turns other than yours, spells you cast cost less to cast."
less to cast."
Jet Medallion80%
$13.42↑#274↑0.2729.0 years
"Black spells you cast cost less to cast."
less to cast."
Ruby Medallion80%
$14.07↑#293↑0.2829.0 years
"Red spells you cast cost less to cast."
less to cast."
Uthros Psionicist78%
$0.22↓#9584↑0.221.2 years↓
"The second spell you cast each turn costs less to cast."
less to cast."
Raging Battle Mouse78%
$0.22↓#12427↑0.00↓3.1 years↓
"The second spell you cast each turn costs less to cast."
less to cast."
Highspire Bell-Ringer78%
$0.16↓#10943↑0.00↓1.5 years↓
"The second spell you cast each turn costs less to cast."
less to cast."
Urza's Filter76%
$4.88↑#14664↑0.30↑26.0 years↓
"Multicolored spells cost less to cast."
less to cast."
Killian, Ink Duelist76%
$0.31↓#3410↑0.285.5 years↓
"Spells you cast that target a creature cost less to cast."
less to cast."
Accursed Witch // Infectious Curse74%
$0.29↓#10803↑0.1410.5 years↓
"Spells your opponents cast that target this creature cost less to cast."
less to cast."
Herald of Kozilek74%
$0.36↓#6409↑0.2911.0 years↓
"Colorless spells you cast cost less to cast."
less to cast."
Cheering Fanatic74%
$0.12↓#202150.00↓8.4 years↓
"Whenever this creature attacks, choose a card name. Spells with the chosen name cost less to cast this turn."
less to cast this turn."
Planar Gate72%
$74.69↑#214340.38↑32.4 years↑
"Creature spells you cast cost less to cast."
less to cast."