This is one of this words that will make sense when used in context. But by itself it’s one of those words that make you think “WTF mate?” It is pronounced just like the word with the same spelling that means “joined together”, which could possibly add to the confusion.
union (n.) IPA Pronunciation: 
pearl
CLAUDIUS
And in the cup an union shall he throw,
Richer than that which four successive kings
In Denmark’s crown have worn….– HAMLET (V.ii)
Again, in context, when you’re seeing a production of Hamlet and Claudius is holding a pearl saying this, the logical conclusion you’ll make is “Ah, union must be a pearl”, OR “I don’t know what the heck he just called that, but it’s a pearl…”