Freedom Emeralds

The freedom emeralds are green gems much like chaos emeralds. They are spherical and opaque. They appear in the sonic special "a crowning achievement" which can be found in sonic archives volume 0. In the special, princess Sally calls Sonic to help her find the freedom emeralds. She explains that they belong on the crown of acorn, and that the four of them keep Dr. Robotnik's evil reign at bay. Since that special, the freedom emeralds have not appeared and were clearly a minor group of gems.