Those earrings must be very heavy to wear but gorgeous to see!
I would have suspected to have worn garnets on a day occasion. Bohemian garnets are quite famous in Prague,

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Maxima was wearing some of the oldest House Diamonds as earrings in Prague last night. These large rose cut diamonds are taken from the House Diamond tiara. Maxima was the first (known) to wear these as pendant earrings.
She was also wearing one of the two bracelets created from Queen Juliana's wedding gift bracelet and an open work diamond bracelet that belonged to Queen Wilhelmina.