In the dictionary,
ransom: money that is paid to somebody so that they will set free a person who is being kept as a prisoner by them
It seems like "ransom" is for people, for example, "They kidnapped the kid for ransom".
Now, a man stole a "17th century vase" but he didn't want to sell it but called the owner of the vase and asked for a ransom.
Is it correct to say "he stole an antique for ransom"?