I think the mother of the groom, Princess Marie of Schwarzburg-Rudolstadt, by then Dowager Grand Duchess of Mecklenburg, was wearing the large Mecklenburg tiara seen on Friedrich Kaulbach's portrait of her late husband's first wife Princess Augusta Reuss of Köstritz.



Augusta Reuss of Köstritz's daughter-in-law, Grand Duchess Anastasia Mikhailovna of Russia, the mother of the young Grand Duke of Mecklenburg-Schwerin seated next to Queen Emma, was also portrayed wearing the tiara.



A website named Unofficial Royalty states that Wilhelmina was wearing the smaller version of the Stuart Tiara which – as Beth has already suggested – seems most likely.