This 1904 tiara, designed by Louis Cartier, was revolutionary because it was one of the first to use platinum.
Prior to this tiaras were made from gold or silver, and platinum was used in industry. Platinum is much stronger than gold, so you can make lighter airy designs, while still holding the stones securely. And platinum looks very good with diamonds.
This tiara also features another innovation, tremblant. The round circular diamonds are suspended on a chain, so they tremble when the wearer moves, catching the light from all directions.