"Do you like my new gems?"
Translation:Placentne tibi novae gemmae meae?
"But here..." I don't see a context, so, of course, each of us can make up a context for this sentence.
It's worth learning, for English speakers, that Latin didn't pepper sentences with "my" and "your" and "his", etc., when the all-important context (the thing not supplied in these discrete sentences!) makes it obvious.
If something plural (like "gems," gemmae) is "pleasing," then the verb will be plural: placent .
If it's just one singular item (like "a gem," gemma ) that is "pleasing," then the verb will be singular: placet .
There are lots of sentences of the type, "It pleases (someone--dative case) to do something (infinitive form of verb)," in which case we use singular verb placet , since the infinitive is considered a singular noun:
"It pleases me to wander in the streets": Mihi placet in viīs errāre .