That bottom picture is the one Ursula uses at her site. Here's what it says about the Derby tiara. The outfit she wears is the one the 8th Duchess wore at the famed Devonshire Ball in 1897 where so many royalties attended in full costume.
"The tiara rests on a band of pairs of stylized buds between collet-set diamonds, surmounted by a sequence of palmettes outlined by diamond borders linked at the base to lotus flowers graduated toward the back,in total ca 1900 diamonds. The design is made in 1802 by Skinner of Orchard Street for the Duchess of Devonshire, who was presumably given this tiara at the time of her marriage in 1889. Since then the tiara has descended through three generations.
An articel written by the Duchess in the Sunday Telegraph from March 17, 2002, described her own behaviour when wearing so much glitter:
“Before the last war, tiaras were worn by married women at all the grand balls in London. Even at a big dance in the 1960s it was not uncommon for men to wear tail-coats and the women their jewels. I remember going to a such an entertainment on my own wearing, with unwonted confidence, the "big" tiara (the Devonshires have two). It must have looked rather odd, because my home-made dress of cotton broderie anglaise was definitely not up to it. At the end of the evening I went out to look for a taxi. It never occurred to me that it might not be a good idea to stand alone in the street long after midnight with a load of diamonds round my neck and nineteen hundred more glittering above my head.
But then, even though Helen, Duchess of Northumberland, once had her tiara snatched off her head as she was leaving her house in Eaton Square, we did not think of being mugged (the word did not exist). My mother-in-law, Mary Devonshire, who was Mistress of the Robes to The Queen from 1953 till 1967, used to fetch the jewels from the bank stowed in a Marks& Spencer carrier bag. My grandmother-in-law, Evelyn, Duchess of Devonshire, was Mistress of the Robes to Queen Mary for 43 years from 1910. Together she and Queen Mary weathered long hours of tiara-ed evenings. After one particularly lengthy engagement, Granny was heard to say "the Queen has been complaining about the weight of her tiara . . . the Queen doesn't know what a heavy tiara is".
Evelyn knew what she was talking about. The larger of the two Devonshire diamond tiaras is indeed a whopper. It was made in 1893 for Louise, the eighth Duke of Devonshire's wife. She was formerly married to the Duke of Manchester and was always known as the Double Duchess..........”source:Sunday Telegraph
In addition to the link in the previous page, this one also contains information on the tiara and more photos:
http://www.royal-magazin.de/england/devonshire/diadem-derby.htmand this one shows the dress and jewels the 2 Duchesses wore (in color):
http://www.royal-magazin.de/england/devonshire/parure-devonshire-holbeinesque.htm"Left the Zenobia-costume from Worth,Paris, which the 8th Duchess of Devonshire,Louise, wore at the famous Devonshire Diamand Jubilee House Ball in the 1897
"...The skirt of gold tissue was embroidered all over in a star-like design in emeralds, sapphires, diamonds, and other jewels outlined with gold, the corners where it opened in front being elaborately wrought in the same jewels and gold to represent peacocks' outspread tails.
This opened to show an underdress of cream crepe de chine, delicately embroidered in silver, gold, and pearls and sprinkled all over with diamonds.
The train, which was attached to the shoulders by two slender points and was fastened at the waist with a large diamond ornament, was a green velvet... and was superbly embroidered in Oriental designs introducing the lotus flower in rubies, sapphires, amethysts, emeralds, and diamonds, with four borderings on contrasting grounds, separated with gold cord.
The train was lined with turquoise satin. The bodice was composed of gold tissue to match the skirt, and diamonds, and the front was of crepe de chine hidden with a stomacher of real diamonds, rubies, and emeralds. Jewelled belt. A golden crown incrusted with emeralds, diamonds and rubies, with a diamond drop at each curved end and two upstanding white ostrich feathers in the middle, and round the front festoons of pearls with a large pear-shaped pearl in the centre falling on the forehead." The Times "