Queen Sofia wore Mellerio Shell Tiara at Royal Wedding. It's obvious that it was made by French Jeweler Mellerio. It was made in mid 19th century. It was displayed at the 1867 Paris Exhibition and the tiara was a wedding present from Queen Isabella II to her daughter on her marriage. I can't trace it but for some reason it came to Spanish Royal family and was a gift to Queen Sofia by her father-in-law on her wedding day.
Thanks to Uschi I am able to trace it :) Here is the information Uschi shared with us :
The Shell Tiara, made of platinum and diamonds forming shells with pear-shaped pearls dangling from their centres, dates back to Queen Isabella II who offered it to her daughter Infanta Maria Isabel on the occasion of her wedding to Prince Gaetano of Bourbon-Two Sicilies in 1868. The Shell Tiara was inherited by her nephew King Alfonso XIII who passed it on to his son Don Juan, Count of Barcelona. He offered the Shell Tiara to his son Don Juan Carlos on the occasion of his wedding to Princess Sofía in 1962.