Forget an iced latte, Stefon Diggs just got an iced out Starbucks chain -- and it's all because the dude is OBSESSED with the coffee house ... TMZ Sports has learned. 

In case you didn't know ... the Minnesota Vikings star WR is a self-proclaimed Starbucks addict (he even wore Starbucks-themed cleats against Seattle last season).

But, Diggs decided to take it one step further ... hitting up Al the Jeweler from Labelle Jewelry in Totowa, NJ to create a diamond-studded tribute to the java joint.

Al tells TMZ Sports the piece -- which features more than 1,000 emeralds and VS diamonds -- took over a month to complete. We're told Diggs loved the finished product more than an espresso macchiato (and who doesn't love those?).

Al -- who works with huge stars like Jarvis Landry and Terrance Williams (to name a few) -- was tight-lipped on how much Diggs spent on the chain ... but our sources estimate it to be around $15k.

Oh, and we're also told one of the reasons Stefon got the piece was because, like Starbucks, he's "open all day."