Enamel and diamond brooch, late 19th century

The oval blue guilloché enamel plaque set with a rose diamond monogram, with the initials RHG for Royal Horse Guards, within a border of cushion-shaped diamonds, inscribed to the reverse, To Lady Castlereagh from Four brother Officers - Windsor, 1901.

Mounted in silver and gold. Estimated total diamond weight approximately 3.50 - 4.00 carats of mixed colours and clarities. Enamel with slight nicks to edges. Scratches to mount consistent with age and wear. Pin a little bent. In good condition.
"In response to your inquiry, we are pleased to provide you with a general report of the condition of the property described above. Please note that colour, clarity and weight of gemstones are statements of opinion only and not statements of fact by Sotheby's. We do not guarantee, and are not responsible for any certificate from a gemological laboratory that may accompany the property. We do not guarantee that watches are in working order. Prospective buyers should inspect each lot to satisfy themselves as to condition and must understand that any statement made by Sotheby's is merely a subjective, qualified opinion. Prospective buyers should also refer to refer to any Important Notices regarding this sale, which are printed in the Sale Catalogue, in particular to the Notice regarding the treatment and condition of gemstones and to the Notice regarding import of Burmese jadeite and rubies into the US.
NOTWITHSTANDING THIS REPORT OR ANY DISCUSSIONS CONCERNING A LOT, ALL LOTS ARE OFFERED AND SOLD "AS IS" IN ACCORDANCE WITH THE CONDITIONS OF BUSINESS PRINTED IN THE SALE CATALOGUE."

Edith Vane-Tempest-Stuart, Marchioness of Londonderry (neé Edith Helen Chaplin), was a famous socialite and noted political hostess. Granddaughter of the Duke of Sutherland, in 1899 she married Lord Castlereagh who became the 7^th Marquess of Londonderry in 1915. During the war, Edith founded the Woman's Legion and was presented with a Military DBE. Her husband served with distinction in his Regiment the Royal Horse Guards, from whom the brooch was given to his wife in 1901 by four of his fellow officers. Lady Mairi Elizabeth Vane-Tempest-Stewart was the youngest of their five children, who became Lady Mairi Bury upon her marriage to Viscount Bury in 1940.