Information on these jewels that pass from monarch to monarch: the joyas de pasar, or, “the jewels that are passed on.”
And more detailed information on the Spanish language Wikipedia version.
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The jewels of passing are a set of jewels gathered by Queen Victoria Eugenia of Spain, linked to the head of the Spanish Royal House, and worn in turn by the queens of Spain.
It was the Countess of Barcelona who coined the expression jewels "to pass", when Queen Victoria Eugenia (born Princess of Battenberg) gave her the historical pieces when her husband, King Alfonso XIII, died...
This brings up a very special, and yet sad, touch to Queen Letizia's using them for King Constantine's funeral.