Sapphire Medallion
Artifact
Blue spells you cast cost 
less to cast.
It is the glimpse of a new horizon beyond the hills, the briny taste of salt spray off a ship's hull, and the whisper of pages turning in a new book. It is discovery.
Daniel Ljunggren
Modern Horizons 3 (MH3)
• #296Rulings
If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell's total cost.
(wotc • 2023-07-28)
If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben's ability), apply those increases before applying cost reductions.
(wotc • 2023-07-28)
The ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
(wotc • 2023-07-28)
The cost reduction can apply to alternative costs such as flashback costs.
(wotc • 2023-07-28)
The ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that cost.
(wotc • 2023-07-28)
Rulings
If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell's total cost.
(wotc • 2023-07-28)
If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben's ability), apply those increases before applying cost reductions.
(wotc • 2023-07-28)
The ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
(wotc • 2023-07-28)
The cost reduction can apply to alternative costs such as flashback costs.
(wotc • 2023-07-28)
The ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that cost.
(wotc • 2023-07-28)