Gold, gems and dust: what each one is, where it comes from and what it is for.
The three currencies
Currency
How you earn it
What it is for
Gold
By playing: duels, quests, daily gift, pass. Never sold.
Traveler's Packs, the showcase (common and uncommon cards), trade fees, tournament entry.
Gems
Bought with money (when purchase is available in your country) and in the premium pass. They do not expire; no cash-out or transfer.
Reliquaries (gem packs), the showcase (rare and epic), cosmetics, characters.
Card dust
By disenchanting cards; also in the Daily Challenge and the premium pass.
Crafting specific cards.
Where gold comes from
Source
Gold per duel
Cap per day (UTC)
Campaign and Free duel (win against the computer, by grade)
20–120
300 gold
Daily Challenge and Legend Mode (same table, same cap)
20–120
inside the cap above
Crossing of Myths (per duel won: normal / elite / Guardian)
10 / 20 / 40
60 per day, inside the cap above
Ranked (win / loss)
50 / 15
300 gold
Casual (win / loss)
25 / 5
inside the PvP cap
On top of that there is gold from quests (Quests and Daily gift), from the Daily gift and from the free pass track (Season pass); those do not count toward the caps above.
Where to spend
Traveler's Packs for 1,500 gold, with 5 cards (Packs).