Lil Pump went from fuming over music festivals disrespecting him to flashing a huge smile, and all it took was a special delivery of diamonds.

Pump was venting Wednesday at LAX ... about having to perform on stages that are way too small for the huge crowds he's drawing, when some luggage arrived to cheer him up.

Celeb jeweler Johnny Dang -- known for making mouth bling for rappers and athletes like the World Series champion Astros -- met Pump at the airport ... and put a whole lotta money where his mouth is.

The "Drug Addicts" rapper says these custom-made grills are worth $70,000 and according to Johnny ... they're flawless.

No word if they made it past TSA.