This huge, traditional round-cut diamond, whose 2.5 cm (1 inch) diameter is equal in size to the visible part of a human eye, bears the same name as the whole Dynasty collection.
According to state-controlled Alrosa, it is potentially the most expensive diamond manufactured in the history of Russian jewelry because of its high quality.
It took a year and a half for Alrosa's specialists to create five polished diamonds for the collection from a 179-carat Romanovs rough diamond, extracted at one of the company's mines in Russia's far eastern Yakutia region in 2015.
The collection is dedicated to the Tsar Peter the Great, who built the city of St Petersburg, and the Romanov dynasty, which ruled Russia for 300 years, Alrosa, the world's largest producer of rough diamonds in carat terms, said in a statement.
The collection's name fits well with a surge in patriotism in Russia triggered by Moscow's annexation of the Crimean Peninsula from Ukraine in 2014 and Western sanctions against Moscow.
"There was a good reason to choose the name for the collection, which is connected with Alrosa's intention to revive the traditions and memory of renowned Russian jewelers famous for their craftsmanship and filigree since Russia's first cutting and polishing factory founded by Peter I (the Great) early in the 18th century," the company said.
In an industry context, Alrosa's decision to produce these polished diamonds and sell them online fits with a broader quest to find new ways to the market and add value on the part of gem producers.
Alrosa and Anglo American's De Beers unit, which for the first time auctioned polished stones this year, produces about half of the world's rough diamonds.
The main 51.38-carat diamond has a 57-facet traditional round brilliant cut with triple excellent cut, D color and VVS1 clarity.
"As to overall characteristics, it is unprecedented in the history of Russia," Alrosa said. The other stones in the collection are a 16.67-carat round-cut diamond, a 5.05-carat oval diamond, a 1.73-carat pear-cut diamond, and a 1.39-carat diamond.
Alrosa did not disclose the estimated value of the main diamond in the collection. Based on the value of other large polished diamonds sold at global auctions, the main gem could be worth at least several tens of millions of dollars. (Reporting by Polina Devitt in Moscow; Additional reporting by Diana Asonova in Moscow and Barbara Lewis in London; Editing by Dale Hudson)