Afrojack didn't just ice up his own neck for his birthday, he made sure his whole crew was shining ... even though it cost him SIX figures! Best. Friend. Ever.

The Dutch DJ ordered 5 chains with custom Afrojack logo pendants made with white gold and diamonds. He bought one for himself, but the other 4 went to his closest friends. A source close to Afrojack tells us it was a thank-you for supporting him during his 10 years in the biz.

We're told each of the 5 pieces cost $20k -- and Afrojack hit up celeb jeweler Ben Baller of IF & Co. because he was impressed by Lil Uzi Vert's new chain. The goodies were delivered last weekend to his 30th bday bash in Holland.

Talk about a happy customer ... we're told Afrojack ordered 5 more for other members of his entourage.

We're saying he's spending like there's "No Tomorrow" -- his new track -- but he's got stacks for days.