I've noticed that she seems to never wear a wedding band. She wears other rings though.
According to Vanitatis magazine:
"Her wedding ring, which disappeared from her hands in 2011 and has not been put back on since, was said at the time to be taken off because it bothered her when shaking hands, even causing injuries."
Letizia has been "wearing a handcrafted ring from Karen Hallam jewelry, large and colorful, for more than a year. It was a gift from Don Felipe to celebrate the day they started dating, according to the Queen herself."
A different story about the oval ring from HOLA!:
"Queen Letizia has signaled a gold oval ring as one of her favorites. She’s been spotted wearing it almost every time she’s out, including during her trip to Cuba. The minimalist ring holds a special place in her heart as it was given to her by her daughters, Leonor and Sofia. The girls saved for some time until they were able to afford the 114 euros ring."