This tiara was given to Princess Beatrix of the Netherlands (who later became Queen Beatrix) on her 18th birthday in 1956.
It's believed to have been made in the early 19th century when there was a revival of a quasi-Roman style, led by Napoleon's court. It's not known if the Dutch royal family already owned it, or whether it was bought especially for Princess Beatrix. In the photographic era, it's only been seen from 1956 onwards.
It then started to be worn as a wedding tiara by lesser members of the royal family, e.g. Prince Constantijn's bride Laurentien wore it.
As you can see below, it's a simple elegant wreath of laurel leaves made of diamonds. It's quite difficult to wear if you are not an early 19th century sylph. Princess Beatrix was too hearty for it.