Crisol translates to "crucible," a vessel used for melting materials at high temperatures. It's a less common, more poetic word that evokes a sense of intense transformation and purification.
In the song, the lyric "el oro sea crisol con el fuego" (the gold becomes a crucible with fire) uses this word metaphorically. It suggests that just as gold is refined and purified by intense heat in a crucible, difficult experiences or the end of a relationship can lead to a similar process of refinement and change. This makes it a powerful and thought-provoking word.
Alexander Acha's "Adiós" is a heartfelt goodbye, but it's not a sad, drawn-out one. It's more of a firm, necessary farewell where both people know it's time to move on. He sings, "Adiós, ya no tiene caso vernos" [Goodbye, there's no point in seeing each other anymore], acknowledging that it's over, but also holding onto the good memories: "nos quedamos con lo bueno" [we'll keep the good stuff].
He wishes his former love well, hoping they find what they're looking for, even if it's with someone else: "que te carguen otros brazos y tu almohada sea otro pecho" [may other arms hold you and your pillow be another chest]. It's a mature, if painful, decision, as he admits, "no es esto lo que quiero, pero sé que es lo correcto" [this isn't what I want, but I know it's the right thing to do]. He knows it's hard to understand, but some things just aren't meant to be, and sometimes you just have to say "adiós."