The pendant is described as "a removable pendant in grey gold, in the shape of a Saint Andrew's cross, studded with old-cut cushion-shaped diamonds".
What do other posters think? Is it the Princess of Joinville's Southern Cross brooch? Or just a later copy of the original brooch?
I am quite sceptical about the auctioned pendant being the Princess of Joinville's brooch, for several reasons: - first, the auction website mentions that the necklace dates back to circa 1920 (though this mention might be for the necklace only, and not the pendant) - the Southern Cross brooch was inherited by the Countess of Paris' youngest daughter, Princess Chantal, Baroness de Sambucy de Sorgue - and as far as I know, Princess Chantal's husband is quite wealthy, so I suspect Princess Chantal and her family are not in need of cash... - the Princess of Joinville's Southern Cross brooch is described in Bernard Morel's comprehensive book about the French Crown Jewels as "a diamond brooch, including 13 main diamonds, mounted on gold and silver", whereas the pendant offered at auction is described as "a pendant in grey gold" - and last but not least, the estimate price (between 13,000 and 18,000 euros) seems to me quite low for such a historic brooch!
Here are pictures of the Princess of Joinville's Southern Cross brooch, worn by the late Countess of Paris (pictures from Lorenzo's website http://diamantsdelacouronne.free.fr/) and by Princess Chantal (picture by myself, taken at the Duke and Duchess of Vendôme's wedding in Senlis in 2009):