On November 24, 1869, Alexandrinsky Square, as Ostrovsky Square was then called, was crowded. In the presence of the imperial family, a monument to the Autocrat of All Russia — Catherine the Second — was laid here. There is a legend that the laying of the monument made such a strong impression on one of the spectators that, not knowing how best to express her emotions, she tore a diamond ring from her finger and threw it into the foundation pit as a token of gratitude to the great empress for her deeds. Noble ladies and gentlemen present at the ceremony followed her example, and soon many earrings, rings, brooches, bracelets, and necklaces lay at the bottom of the pit. The monument laying ceremony lasted longer than planned because many wished to "offer as a gift" their precious jewelry to Catherine II.

Rumor has it that the treasures still lie beneath the monument in Catherine Garden. It has not yet been possible to verify this version. Although during Soviet times, party officials several times had the idea to conduct excavations in Catherine Garden, the plans were never realized.

Sources:

http://www.peterburg.biz/samyie-tainstvennyie-kladyi-sankt-peterburga.html

https://gorodovoy.ru/news/vo-vsem-mire-rabotayut-chto-zasluzhit-premiyu-i-tolko-v-peterburge-chtoby-ee-ne-lishili-kto-iz-gorozhan-ne-poluchaet-nadbavku_id892384_a177_dp250818