Gems are Tarc's in-app virtual currency, spent in the shop (Tarc Souq) on cosmetic items and temporary boosts.
Gems are earned the same way XP is — by completing lessons, quizzes, daily quests, and login rewards. There is no way to purchase gems with real money; they can only be earned through play.
None of these purchases affect your ability to complete lessons or progress through a course — they're entirely cosmetic or convenience items.
Tarc tracks both your current spendable gem balance and your lifetime total gems ever earned (which doesn't decrease when you spend). Gem-related achievements are based on the lifetime total, not your current balance — so spending gems doesn't undo progress toward those achievements.
The shop includes a "Redeem a Code" option for entering promotional codes, which can grant gems or other rewards directly.
No — gems can only be earned by completing lessons, quizzes, quests, and login rewards. There is no real-money purchase anywhere in Tarc.
No — those achievements are based on your lifetime total gems earned, which never decreases, not your current spendable balance.
Cosmetic avatar frames, profile titles, badges, and temporary XP boosts — nothing that gates lesson content or course progress.
Promotional codes you can enter in the shop's "Redeem a Code" option to receive gems or other rewards directly.