Tom Kaulitz Custom-Designed Heidi Klum's Engagement Ring: 'He Didn't Just Go To a Store,' Says Source

When it came time to pick out an engagement ring for his girlfriend, model and America’s Got Talent judge, Heidi Klum, German musician Tom Kaulitz, knew it had to be special. So he got to work.

“Tom drew the dream ring for Heidi and then went in search of the right stones,” a friend of Klum’s tells PEOPLE. “He didn’t just go to the store.”

“He knew Heidi’s favorite color was green, so he searched for the perfect green stone, an alexandrite, and then found the perfect diamonds to accentuate it,” says the friend.

Another reason that an alexandrite is so special? It changes color. The gem is green in sunlight and can appear red in lamplight making it that much more unique.

Kaulitz, 29, planned the surprise for Christmas Eve morning, and made sure Klum’s four children, Leni, 14, Henry, 13, Jonah, 12, and Lou, 9, played a part.

That morning, Kaulitz and the four kids surprised Klum, 45, with a breakfast tray, with coffee, flowers and a ring box, in her bedroom and he proposed.

“She said ‘Yes’ right away!” says the friend.

Klum announced the happy new with an Instagram post, showing off her custom-made ring, along with the message in caps: “I SAID YES.”

“Heidi loved her ring so much,” says the friend. “It was truly made with love by Tom from beginning to end.”

• For more on Heidi Klum and Tom Kaulitz’s engagement, pick up this week’s issue of PEOPLE, on stands Friday

It will be the third marriage for Klum. “Her kids and everyone around her can feel the happiness and love coming from her,” says the friend.

The couple is “really enjoying their engagement,” says the friend. “Heidi believes in going with your heart and your gut and Tom is it!”