Patriot Planter, Fenton
Fenton made this blue 4-inch square planter to celebrate the 1976 U.S. bicentennial. They were limited in production, the mold being destroyed afterwards. There are images on the 4 sides of George Washington, John Adams, Thomas Jefferson, and Benjamin Franklin. They don't show up too often, but still only sell in the $15 to $20 range. I've seen references to amethyst examples, but if there are any in this color, I've never seen one.
After I made the above comment on amethyst Patriot Planters, Doug McIntosh sent me this photo of one. So they do exist.

In January 2005, one of these amethyst examples sold on eBay for $15.