Learn French: Le rubis et le saphir auront gardé leur éclat. - The ruby and the sapphire will have kept their brilliance.

Auront gardé is the futur antérieur (future perfect) tense. It translates directly to 'will have kept' and emphasizes that by a certain point in the future, the state of keeping their brilliance will already be fully established or completed.

Why do we use leur instead of leurs since there are two stones?

The possessive adjective leur agrees in number with the noun it modifies, which is the singular éclat (brilliance). Even though the ruby and sapphire are plural owners, the trait they possess is treated as a single, shared quality here.

Could auront gardé imply something other than a strictly future event here?

Yes! In French, the futur antérieur is often used to express a strong probability or a logical assumption about a past event, translating roughly to 'must have kept.' Without more context, the literal future perfect 'will have kept' is the most direct translation, but using it for past assumptions is a great B2-level nuance to recognize.

Are gemstone names generally masculine in French?

Yes, most precious stones are masculine, like le rubis, le saphir, and le diamant (diamond). A common exception a B2 learner should remember is l'émeraude (emerald), which is feminine.