The 24-year-old singer popped the question to the 21-year-old model last week while the couple were on holiday on the idyllic islands, and he tried to pick out the "perfect diamond" for the $500,000 sparkler.
Private jewellers Solow & Co., Inc said: "He helped us design the ring himself, and he wanted it to perfectly compliment the shape of her hand, which we were able to achieve. The diamond has a beautiful shape and accentuates her hand wonderfully.
"Justin trusted us to pick out the perfect diamond, and when he saw the completed ring he was thrilled with the results.
"We have known and worked with Justin for over 10 years and were introduced to him through his manager Scooter, whom we have also done work with over the years."
The 'Never Say Never' hitmaker was said to be "excited" to present Hailey with the engagement ring after having a hand in designing the band. Solow added to Hollywood Life: "Justin was thrilled with the execution of the ring we made for his girlfriend at the time, and was excited to give it to her."
Justin and Hailey are said to be planning to have an "intimate" wedding in the singer's native Canada. A source recently said: "They are discussing having a very small wedding.
"They want something private, intimate. Justin loves Canada and is at home there. It's a special place for him and he'd love to get married there."
Justin confirmed the pair's engagement earlier this week by taking to Instagram to post an emotional message. He wrote: "Was gonna wait a while to say anything but word travels fast, listen plain and simple Hailey I am soooo in love with everything about you! So committed to spending my life getting to know every single part of you loving you patiently and kindLY. I promise to lead our family with honor and integrity letting Jesus through his Holy Spirit guide us in everything we do and every decision we make. My heart is COMPLETELY and FULLY YOURS and I will ALWAYS put you first!
"You are the love of my life Hailey Baldwin and I wouldn’t want to spend it with anybody else. You make me so much better and we compliment eachother so well!! Can’t wait for the best season of life yet!. It’s funny because now with you everything seems to make sense! The thing I am most excited for is that my little brother and sister get to see another healthy stable marriage and look for the same!!! Gods timing really is literally perfect, we got engaged on the seventh day of the seventh month, the number seven is the number of spiritual perfection, it’s true GOOGLE IT! Isn’t that nuts?
"By the way I didn’t plan that, anyways My goodness does feel good to have our future secured! WERE GONNA BE BETTER AT 70 BABY HERE WE GO! “He who finds a wife finds a good thing and obtains FAVOR from the Lord!” This is the year of favor!!!! (sic)"