Sean Kingston -- Diamond Guy Cuts Him Down to Size in Court

Sean Kingston just bought some expensive jewelry ... against his will. Let's just say it's a court-ordered purchase.

Sean skated off with $226k worth of jewelry from 2008-2013 but never paid a dime ... this according to Avi Da Jeweler who filed a lawsuit against Sean.

The singer's rep told us last August ... Sean coughed up more than $300k and had the cashed checks to prove it.

If that were so, it should have been easy to win the lawsuit, but Sean never filed an answer so the judge just issued a default judgment, to the tune of $356k.

As puzzling as it is that Sean never produced the checks, it's just as puzzling why Avi kept giving the dude jewelry when he wasn't paying jack.

We've reached out to his rep but so far no comment.