Experts believe the Timur ruby, which is really a translucent stone called a spinel, once regarded in the East as more precious than diamond, has not been seen in public since the Great Exhibition of 1851.
As well as the ruby, the Queen is loaning a belt of emeralds, pearls and diamonds, seized by the British from the Punjab in the mid-19th century.
She is also lending the original gold enamelled setting of the legendary Koh-i-noor diamond - although not the diamond itself - for the exhibition which opens at the Victoria and Albert Museum in London later this month. A spokesman for the Queen said: "These are part of the royal collection, but they don't often see the light of day, because of their nature and the requirements for showing them."
The Timur ruby gained its name from a spurious association with the Central Asian ruler Timur, known in the West as Tamburlaine. It was really the possession of Shah Jahan, whose name is inscribed upon it.
The jewel, like the Koh-i-noor diamond and its setting, was later acquired by Ranjit Singh, the greatest Sikh maharaja of the Punjab, whose court in Lahore was a scene of dazzling brilliance last century.
Susan Stronge, the curator, expects the pieces to revitalise the Maharaja's image. Respected by the British for his military prowess, he was dubbed the "Lion of the Punjab". But Ms Stronge believes the exhibition will reveal the magnificence of his court and his role as an avid collector of treasures. "Many people think the Sikh court was not very interesting, but it was really splendid," she said.
A 19th-century visitor, Henry Edward Fane, wrote: "The dresses and jewels of the Rajah's court were the most superb that can be conceived; the whole scene can only be compared to a gala night at the Opera."
The exhibition has at its core the V&A's own collection of textiles and one of its most prized pieces, the Golden Throne of Ranjit Singh. The Sikh community, led by the Sikh Foundation of America, has provided works from private collections and museums in India, Pakistan, Europe and North America. It marks the 300th anniversary of the Khalsa, one of the most important events in Sikh history when the visual symbols associated with Sikhs - such as uncut hair for men - were prescribed.
Among other exhibits that the Queen is lending the V&A are photographs and paintings of Maharaja Dalip Singh. The youngest son of one of Ranjit Singh's queens and the last Sikh ruler of the Punjab, he came to Britain after the British annexed the region in 1849 and was befriended by Queen Victoria.