The 39-year-old singer and fashion designer reflected on her "unity" with Johnson, a former professional football player, while celebrating their wedding anniversary Sunday.
Simpson shared a photo of herself and Johnson holding up her gift. Johnson gave Simpson a large geode crystal in the shape of a butterfly.
"Eric Johnson, my Husband, I love you," Simpson captioned the post. "6yrs ago today I married my perfect soulmate. Our unity was written in God's sky of colliding stars."
"Together we manifest dreams, nurture desire, and hold space. Destined, it always has been and always will be, forevermore," she said. "Also, anyone who knows me at all knows that this crystal butterfly is my dream gift."
Simpson and Johnson married in 2014 and have three children, son Ace, 7 and daughters Maxwell, 8, and Birdie, 15 months. Simpson shared a new photo of Johnson and their kids while celebrating Father's Day in June.
"My husband's most beautiful quality is being the most thoughtful father in the universe," the star wrote. "He gives complete attention to what they do and what they need. He is present in EVERY moment keeping them safe, nurtured, and loved."
"There is no greater gift to this family than Eric Johnson," she said. "You are all our best friend, and we are yours. We are blessed, so very blessed my love. Happy Father's Day to the GREATEST there is."
In May, Simpson celebrated her 10th anniversary as a couple with Johnson. She previously said she considers the day she met Johnson as the anniversary of when they started dating.
"I fell head over slippers in love with this perfect man 10 YEARS ago today! By chance he knocked on my dreamy cottage door (sigh), I let him in and never let him leave," Simpson said May 21 on Instagram. "He is mine, I am his... forever #MAY212010."
Simpson was previously married to Nick Lachey, while Johnson was wed to Keri D'Angelo.