The description gives further details, mentioning as provenance Margherita of Savoy, Queen of Italy (1851-1926), wife of King Umberto I, and Elena of Montenegro, Queen of Italy (1873-1952), wife of King Vittorio-Emmanuele III. The catalogue notes further add that "according to family tradition Umberto I, King of Italy (1844-1900) gifted this brooch to his consort Margherita of Savoy, Queen of Italy (1851-1926)".
Unfortunately, Sotheby's does not provide us with a picture of the brooch worn by a model (as they frequently do now), which would have enabled us to get a better idea of the brooch's size.
The current seller of this brooch is not identified - notably, it is not said if the seller is a member of the Savoy family (and who), or if it was already sold years ago by the Royal family and is now sold by someone else.
Unfortunately, I have never seen any clear picture of any Savoy Queen or Princess wearing this brooch.
Yet, I wonder (without any certainty, though) if this brooch could be the clasp of one of Queen Margherita's bracelets on the following pictures (on the first picture, I am aware that the tiara, necklace and corsage brooches are made of emeralds, but as the bracelet clasps look paler, I wonder if they could be sapphires):
To make the plot thicker, another very similar brooch, also from the Savoy Royal family, was auctioned off in 2009. Very similar, but different anyway (as you can see on the pictures above and below, the shape of both sapphires differs, as well as the details of the diamond motifs of the outer surround). The brooch sold in 2009 was reported to have been made around 1910 for Queen Elena (wife of King Vittorio-Emmanuele III). It was sold at auction by Tajan in Monte-Carlo on 28th July, 2009, as the centrepiece of a pearl choker:
According to Vincent Meylan in Point de vue at the time of the 2009 sale, the brooch had been already sold at auction previously, "at the end of the 20th century" in London (but I could not manage to find back when exactly).
Queen Elena's brooch was pictured with other sapphires jewels belonging to Queen Marie-Josée, and it was also pictured in Maria-Gabriella di Savoia and Stefano Papi's book Gioielli di Casa Savoia (page 97).
Re: Italian royal brooch to be auctioned at Sotheby's