TMZ Sports has learned ... it all came to be when the Memphis Grizzlies superstar was talking to an owner at Ice Box on Instagram Live about making him a one-of-a-kind diamond watch.

During the convo, the two came up with the perfect idea -- not only would the hooper get a watch, but he'd also get a custom chain to match!!

The brainstorming sesh resulted in Morant's new black and white diamond Miami Cuban chain with an iced-out "12" clamp and pendant ... representing Ja's jersey number.

Morant -- who calls himself "12SKII" -- also got the word SKII on the bail of the pendant.

We're told the pendant comes with 20 carats of black and white VS quality diamonds and the Cuban link has 60 carats. Both pieces are made with 14-karat white gold and weigh an overall 400 grams.

As for the 22-year-old's new watch ... we're told it's worth over $100k and fully customized to Ja's liking with black diamonds.

Of course, buying jewelry from the Ice Box in Atlanta is a hobby for Morant -- he just bought some bling in August while congratulating a newlywed couple.

In April, he went on a $55k shopping spree and took some tequila shots with rapper, Gunna.