As usual with false friends, these words have similar underlying ideas, but are not the same.

The English word “rare”, much like its Spanish lookalike, means that something is infrequent, perhaps even a little special.  But it’s not unusual.   And that’s what makes it different from the Spanish word.  A more common translation for the Spanish word “raro” is “strange”, which looks a lot like “extraño”, because they both come from the same root.  Check out these examples:

Rare:

This almost flawless diamond is a rare discovery; most have many flaws.
I rarely get up before 10AM on Saturdays.
Ten years ago, foreign tourists were a rare sight in this city—now we get many!

Strange:

It’s very strange to see Bill down here—normally the bosses avoid visiting this area.
It’s strange to see Mike wake up so early, since he almost always sleeps until 2PM—I’m worried.
It’s so strange to see how much the town has changed in the ten years I’ve been away.