This might be the reason Tekashi69 keeps his cool with bullets flying around him almost constantly ... his mouth is full of ice! 

The rapper hit up celeb jeweler Johnny Dang for his newest pair ... and the chompers are off the chain. We're talking 18-karat white gold with 585 diamonds in a special honeycomb setting.

This smile on the rocks cost 6ix9ine a cool $45,000. 

They're Tekashi's first grills since he got veneers ... so he had to get a new molding made, and Johnny went the extra mile to ensure the rapper's mouth shines like the sun.

Johnny met Tekashi in Los Angeles Monday, traveled home to Houston and finished up the grills in less than 24 hours, and then flew back out to the West Coast Thursday for a special delivery. 

It's interesting ... Tekashi got the grills hours before his music video shoot with Kanye West and Nicki Minaj was halted by gunfire ... and, we're guessing the mouth full of ice helped Teskashi6ix9ine keep his cool under fire.

Sorry for saying that twice.