Learn Spanish (Spain): La anticuaria tasó este collar ayer y, en efecto, es valioso. - The antique dealer appraised this necklace yesterday and, indeed, it is valuable.

en efecto translates to "indeed" or "sure enough" and is used to confirm an expectation or suspicion. In this sentence, it implies the speaker already suspected the necklace was worth a lot, and the appraisal confirmed it. de hecho (in fact) is usually used to add new, reinforcing, or even surprising information rather than just confirming a suspicion.

Could we use the present perfect ha tasado instead of the preterite tasó here?

Not in Spain. The word ayer (yesterday) places the action in a closed timeframe that is completely disconnected from the present. In Spanish from Spain, you must use the preterite (tasó) for finished, past timeframes, saving the present perfect for timeframes that include today (like este mes or hoy).

Why does the sentence use es valioso instead of está valioso, since the value was just confirmed yesterday?

Even though the value was recently discovered or confirmed by the appraiser, being valuable is considered an inherent characteristic of the necklace, not a temporary state. Therefore, we use ser (es) rather than estar.

Is valioso a false friend for English speakers?

It can be if you confuse it with "valiant" or "brave", which translates to valiente in Spanish. valioso strictly means "valuable" or "precious", referring to the worth of an object or contribution.