TMZ Sports has learned the ring, which SCP Auctions describes as a "fine piece of jewelry," weighs 65 grams and is 14-karat gold with 1.625 carats of real diamonds.

The top of the huge, 12 3/4 sized ring resembles a basketball, and has the words "Naismith Basketball Hall of Fame" encircling the brown enamelwork.

On one side of the ring is "William F. Russell" ... with the year "1975," when he was inducted.

On the other side is the word "Player" -- which Russell embodied throughout his career -- which sits above an image of Dr. James Naismith, the man credited with inventing basketball.

The ring also comes with a letter describing its rich history and provenance ... signed by the GOAT himself.

Remember, Russell famously declined to attend his induction ceremony in '75 ... saying he refused to accept the honor 'cause he felt others before him should be inducted first.

Instead, Russell -- a 13-time champ (11 as a player, 2 as a player-coach) -- opted to receive his ring in 2019 at a private ceremony with his wife, Jeannine, and close friends.

The Celtics legend passed away in July at age 88 ... and was celebrated through the sports world for his contributions to the game of basketball both on and off the court.

The NBA has since listened to the pleas of Hall of Famer Magic Johnson, who -- in the aftermath of Russell's death -- advocated the league retire Bill's iconic No. 6.

Russell's No. 6 will be retired throughout the league, making him the first player ever to receive the honor ... fitting considering his legacy and impact.

The ring has already been bid on 5 times ... with a high bid of $43,923.