Quoting in part from 'Queen Mary': James Pope-Hennessey.

A Durbar was held in Delhi on three occasions during the British Raj. In 1877, for the Proclamation of Queen Victoria as Empress of India, which was presided over by the Viceroy, the first Earl of Lytton. In 1903, for the Coronation of King Edward VII, presided over by the Viceroy, the first Earl Curzon of Kedleston (created Marquess in 1921). And in 1911, for the Coronation of King George V, over which the King presided in person.

It was King George himself who had suggested that he should go out to India to hold his Coronation Durbar in person. When he had first broached the subject to the Prime Minister in September 1910 it aroused no enthusiasm in the Cabinet. 'It was entirely my own idea to hold the Coronation Durbar at Delhi in person', the King wrote in his Diary,' and at first I met much opposition.' This opposition took several forms: objections on the ground of expense, for was it not both unwise and prodigal to hold so pompous a public ceremony at the moment when large parts of India were, as usual, in the grip of famine? There were also grave objections on the grond of security risks, for in the six years which separated King George's accession from his visit as Prince of Wales in 1905, anti-British revolutionary movements had made great headway there. Then there was the question fo how [the United Kingdom] would get along without its King for a period of three whole months. But the King would not be deflected: his experiences in India in 1905 had convinced him that bureaucratic government was unsuited to India, and he hoped by his personal appearance in Delhi to strengthen and re-emphasise the powers of the hereditary princes and ruling chiefs, whom he persisted in regarding as beneficent and paternal influences. In the end the Cabinet gave way, and active preparations for the Durbar commenced in London and in India. King George V thus became the first English Monarch to visit the East since Richard Coeur de Lion, and the only British King-Emperor to visit his Indian dominions in imperial state.

Once the decision for the King-Emperor, accompanied by the Queen-Empress - for Queen Mary had soon scotched a plan to leave her behind in England as Queen-Regent - to go to India had been taken, a litter of fresh problems raised their heads. First of all, what was the King-Emperor to do at Delhi once he got there? Was he to be crowned at a second Coronation ceremony? Was he to crown himself? The King personally favoured what Lord Crewe [Secretary of State for India] called 'a Napoleonic auto-Coronation', but to this there were two insuperable objections: on the one hand the Archbishop of Canterbury pronounced that even an auto-Coronation required a Christian religious ceremony, and that it would be absurd to hold a Christian religious ceremony before an audience almost wholly composed of Mohammedans and Hindus; on the other hand Lord Crewe, supported by the Cabinet, explained that it would be a dangerous precedent to have a separate Indian Coronation, since it might lead, in after years, to the assumption that a King of England was not Emperor of India until he had been crowned at Delhi. Yet some ceremony there must be: 'One's instinct', wrote Lord Crewe to the Viceroy, Lord Hardinge, 'is to avoid the theatrical, but it does not follow that the instinct is sound, as we have got to impress the people of India, not some more or less cultivated persons over here.' Crewe suggested that the King should at a given moment advance to the front of the arena, receive the crown from the Viceroy who would be holding it up on a velvet cushion, and place it on his own head to the sound of massed bands followed by a general salute. 'The Crown would be brought from here,' Crewe added, 'what Crown it would be is a matter for future decision.'

It was finally decided that the King-Emperor would enter the arena already crowned, with the Queen-Empress, wearing her 'best diadem' on top of a crimson velvet cap of maintenance, at his side. So far so good; until it was discovered that it was illegal to take any part of the Regalia, let alone the crown itself, out of the realm. Sir Walter Lawrence then suggested that a special Indian crown should be made in London, taken out to India and left there. The Indian princes, Sir Walter pointed out, would be only too glad to give individual jewels, thus reducing pro rata the estimated cost of a new crown, which worked out at some £60,000. Crewe and the Cabinet, foreseeing and dreading this as a precedent for a multiplicity of local crowns all over the Empire, vetoed the plan. The crown must be made in London, taken to India, brought bacak again and broken up. To this the King sensibly replied that such a course would offend Indian visitors to London, who would be looking forward to seeing the Indian crown sparkling amidst the rest of the Regalia in the Tower. The result of all these discussions was a compromise: a new Indian crown was constructed by Garrard's, taken out to India on the Medina in a special safe, placed on the King's head in private in his tent and publicly worn by him throughout the Durbar ceremonies. In was a noble, but not a light, crown. 'Rather tired after wearing the Crown for three-and-a-half hours,' the King wrote in his Diary for 12 December 1911, 'it hurt my head, as it is pretty heavy.' The Crown of India was then brought back to London and placed in the Jewel-house of the Tower where it may be seen today.

Queen Mary's 'best diadem' was also made for this occasion.

It was part of the Queen’s parure of emeralds and diamonds made for the occasion by Garrard & Co. Ltd. which also included a necklace, stomacher, brooch and earrings.

The tiara takes the form of a tall circlet of lyres and S-scrolls, linked by festoons of rose and brilliant-cut diamonds.

The upper border was originally set with ten of the Cambridge emeralds, acquired by Queen Mary in 1910 and originally owned by her grandmother the Duchess of Cambridge, but these were removed by 1922 for use elsewhere.

In the year following the Delhi Durbar, the tiara was altered to take either or both of the two Lesser Stars of Africa – Cullinan III and IV; the drop-shaped stone was held at the top of the jewel and the cushion-shaped stone hung in the oval aperture below.

Queen Mary lent the tiara to Queen Elizabeth in 1946 for the 1947 South African Tour and it remained with her until her death in 2002. In 2005, it was lent by The Queen to The Duchess of Cornwall.