Pixie Diamonds are a special payment feature released in 2011 for Pixie Hollow (online game). They allow players whom do not pay a membership to access features such as paying for outfits or furniture. This feature does NOT always require payment, but does if you wish to gain a bunch of diamonds, such as 100.
Different allowance amounts depending on the Membership Plan:
Weekly: 30 Diamonds a week
Monthly: 50 Diamonds a month
Semi-Annual: 120 Diamonds a month
Annual: 240 Diamonds a month