Learn French: Le saphir a un éclat parfait, d'où sa valeur inestimable. - The sapphire has a perfect brilliance, hence its priceless value.

Literally, d'où does mean "from where." However, when you use it to link a fact to its logical consequence, it takes on the figurative meaning of "hence," "therefore," or "which explains." In this sentence, the perfect brilliance is the "place" from which the high value originates.

Why isn't there a verb after d'où? Could I say "d'où vient sa valeur"?

You could absolutely say d'où vient sa valeur inestimable (from where its priceless value comes). However, dropping the verb and just using d'où followed immediately by a noun is a very common, elegant shortcut in French. It works exactly like using "hence" + noun in English.

Why do we use the feminine "sa valeur" if the sapphire (le saphir) is masculine?

This is a classic trap that's easy to forget! In French, possessive adjectives always agree with the noun being possessed, not the owner. Because valeur (value) is a feminine word, it must be sa valeur, even though the object possessing that value (le saphir) is masculine.

Is inestimable the best word for "priceless" in everyday French?

Yes. While its English cognate "inestimable" can sound quite formal or literary, inestimable is the standard French adjective to describe something that is "priceless" or "invaluable" (literally: unable to be estimated because the worth is so high).