Long Island has a true gem in jiujitsu star Diamond Armello. 

The Smithtown 16-year-old has a sparkling future ahead as the world’s second-ranked teen in the martial art, with a record that speaks for itself.

“I’ve had over 400 fights — and it’s over 400 wins,” Armello told The Post after a training session at Vanguard Academy in Lake Grove.

“I think I’ve lost maybe 12 times, or 20,” added the 5-foot-6, 130-pounder. 

Mighty Armello — opponents fear his merciless armbar — has earned the respect of the combat sports community, notably from Long Island UFC star Matt “Steamrolla” Frevola, who trains with the teenager.

“This kid keeps me on my toes. … I always try not to use my strength and size on him, but then again, he’s so good sometimes I have to,” said the 155-pound Frevola, 36.

“He’s in here every day, working his a– off.”

Diamond in the rough 

Armello first hit the mats at 11, and he was promoted to adult classes just months into learning jiujitsu. 

He came in with a chip on his shoulder and said priority No. 1 was earning the respect of his older peers.

“It took a while for them, but they eventually realized that I can handle myself,” Armello said. 

Things are at the point now where other fighters often come to him for advice or to watch him do a move right. 

“Even though I’m the youngest, I know a lot of stuff … normally, you can’t get your blue belt until the day you become 16. I got mine 10 months early.”

There was something else Armello had been grappling with as a tween that led him to pursue jiujitsu and ultimately helped him discover his unrivaled skills.

“I used to get picked on a lot. I wanted to learn how to defend myself if I ever needed to,” he said.

Armello ultimately learned peace through the art of war and never had to lay a hand on a bully. Dopey teens have been wise to back off after he gave a verbal warning of what could happen next. 

Armello attributes restraint and maturity beyond his years to what he learns in the studio.

“I keep my self-control — a lot of self-control,” he said. 

Time to shine 

Companions and training partners at Vanguard have become an incredibly positive influence on Armello’s life and a major reason he can rise above teenage sophomoric behavior. 

“I’ve made a lot of really cool friends here,” he said.

The role models he rolls with have also encouraged Diamond to keep doing the right things outside the room, and he’s proudly taken the discipline principles of jiujitsu into the classrooms of Smithtown East as well.

“When I look back, I’m like, ‘wow,’ where I came from academically. I used to be a C, D student. Now I’m an A, B student,” said the math lover.

“And, if I don’t study, my mom’s not going to let me come to jiujitsu,” he said, laughing. 

The rising 11th grader already has a succession plan laid out for after high school, starting with entering the UFC at age 18. 

Similar to fellow Long Island fighters Aljamain Sterling and Al Iaquinta, Armello plans to flip houses as a real estate agent on the side. 

“I also plan on opening my own gym,” Armello said. “I can’t wait to give back to my community.” 

When it comes time to debut in the Octagon, he knows who to call to be ready.

“I’ve definitely got to keep working with Steamrolla on striking to get to that UFC level,” Armello said. “He always brings the pain to us.”