Japanese Princesses usually have a new parure consisting of a diamond tiara, necklace, brooch and earrings made for them when they turn 20. Women don't inherit the throne in Japan, and they lose their royal status if they marry commoners rather than aristocracy, at which point they return their tiaras to the Imperial family.
As a result there are eight unused tiaras sitting in the Japanese royal family's vaults.
So when Princess Aiko, the only daughter of the current Emperor turned 20, they decided to re-use an old tiara; the one made in 1990 by Mikimoto for her aunt Sayoko who no longer uses it as she has married a commoner.
Here is Princess Aiko wearing the set: