is the d20 product and general setting book.Amethyst: Foundations
is the 4E product and general setting book.Amethyst: Evolution
is a 4E expansion.Amethyst: Hearts of Chaos
is a 4E adventure.
Amethyst: Foundations and Amethyst: Hearts of Chaos were published by Goodman Games. I don't know if they had anything to do with the print version of Amethyst D20. From my understanding Dias Ex Machina Games did the product but used Goodman to get it to print. Goodman didn't want to do an entire line so they agree to just do the main book. I don't know too much about the setting or what other products there are if any. I liked the idea but reading more about it turned me off. (I wanted the setting to use 4E races instead of having their own races. It makes sense since they were just converting the d20 setting but it decreased my interest.)