Camilla's mysterious gems. A black tin trunk. A child out of wedlock. And one of the most remarkable hoards of royal jewellery ever seen. (And it was all purchased with bottles of McEwen's beer...)

In the Autumn of 1942, in the middle of the war, a black metal trunk initialled with the letters MHG arrived at Buckingham Palace. 

Nothing much to look at, the box was addressed to Queen Elizabeth, Queen Consort of George VI (and later the Queen Mother).

There was perhaps a small clue in the decidedly upmarket address, 'Polesdon Lacey, Dorking', marked upon the trunk.

Even so, the contents must surely have caused an intake of breath from whoever had the pleasure of lifting the lid. 

For inside the tin box lay a collection of jewels so beautiful that, today, several of them rank among the most frequently worn  pieces in the possession of the Royal Family.

The sparkling contents were all the more remarkable for what they represent - a happy end to a story marred by Victorian prudery. And for having their origins in bottles of McEwen's beer.

As it happens, the collection has also served up the answer to the riddle of the mysterious 'stomacher' or corsage worn by Queen Camilla at the recent diplomatic reception at the Palace.

Jewellery experts were stumped, but I can reveal that this piece, almost certainly Cartier, was among the many fished out from the that trunk in 1942.

The source of the legacy had been the late Margaret Helen Greville, a friend of Queen Mary, mother-in-law to Elizabeth, Queen Consort and grandmother to the late Queen Elizabeth II. 

Margaret Greville had also been great friends with Alice Keppel, mistress to Edward VII. She was also godmother to Sonia Keppel – Queen Camilla’s grandmother.

Today, she is remembered as a philanthropist, yet her rise through the social hierarchy had been far from smooth. 

Indeed, King George V had been unhappy at his wife's connection with Margaret Greville, an unease provoked by her complicated and decidedly unorthodox personal history.

Margaret Greville’s wealth was first acquired by her father, William McEwan, the Scottish brewer whose company made the bottles of beer still bearing his name today.

Yet ignominy had beckoned when he fathered a child with a servant girl called Helen Anderson.

True, McEwan was a bachelor had had no family to betray, but the child was out of wedlock and this was the 19th century.

So, McEwen paid for a brewery employee helpfully named William Anderson  to pretend to be Helen's husband and, in 1863, a daughter was born in London in 1863. 

She was named Margaret.

After the birth, Anderson went back to his wife and children in Edinburgh and his job in the brewery.

Helen and baby Margaret remained in London until an appropriate interval had passed and it was judged appropriate to return north, Helen as a ‘widow’.

Unlike many such stories of the time, this one had a happy ending for, when the child reached the age of 21, the former servant girl, her mother, married the wealthy brewer. 

Margaret became his step-daughter and sole heiress - as he happily told all who mattered.

In the late 1880s, the McEwens moved to a large townhouse between Park Lane and Piccadilly and Maggie became engaged to the dashing Ronald Greville, son of Lord Greville, Gladstone's Private Secretary - and a friend of the gregarious Prince of Wales, later Edward VII.

Despite a beginning of great difficulty, she led a gilded life - literally so, in some respects. 

The April 1891 issue of Vanity Fair listed wedding presents including a tiara composed of three rivière necklaces, with five diamonds the size of sixpences set down the centre and a diamond aigrette (a head piece designed as egret's feathers) measuring 18-inches and set with nine brilliant-cut diamonds.

This was merely the beginning of Maggie Greville's spectacular collection. Soon, she would be an important client of Boucheron in Paris which made, for example, what we now know as the Greville Tiara

In 1887, Boucheron bought at auction, a large quantity of jewellery which had once belonged to the French Royal Family. 

In 1901, made the first version of this majestic piece.

Originally created to be worn as a crown on the centre of the head with a papyrus leaf design, it was underwent two major changes.

In 1919 it was enlarged so could be worn lower on the head. Then in 1921, it was completely dismantled to be reset in the fashionable Art Deco geometric style we recognise today.

In 1929, Margaret Greville visited Cartier London for a two-row diamond necklace. Nine years later she ordered a matching three-row necklace to make it even more magnificent and what we now know as The Greville Festoon Necklace. 

Francesca Cartier Brickell, author of The Cartiers and granddaughter of Jean-Jacques Cartier, told me that the 1930s with its Royal Jubilee and coronation was a decade of big necklaces for Cartier London.

Cartier also made Maggie's fabulous Chandelier Earrings, worn by the late Queen Elizabeth on a number of occasion and worn by the Princess of Wales at last week's diplomatic reception.

Margaret Greville bought emeralds that had belonged to Empress Josephine, diamonds from Marie Antoinette and a diamond ring that had belonged to Catherine the Great.

She had pearl necklaces that would be restrung annually, and, according to Pam Burridge's biography, she once boasted she ‘would buy the whole of Piccadilly’.

By the time of her death in 1942 and her bequest, Margaret Greville had already mended her relationship with George V.

No doubt it helped when, at one point, she promised to leave her spectacular house, Polesden Lacey in Surrey, to their second son Bertie, the then Duke of York (who became George VI). 

In the end, the house was left to the National Trust as Bertie, by now the King, hardly wanted for property.

But what woman ever claims to have too much jewellery? 

Certainly not Queen Elizabeth the Queen Mother, or those who have followed her in enjoying - and displaying - the generosity of  Margaret Greville. 

• Josie Goodbody is a jewellery writer and author of mystery novels