|This variety should be called Nipomo marcasite agate. It comes from Nipomo, California. It is actually a marcasite deposit that was covered by clear agate hiding the marcasite within.
All caps cut by the author. The first cabochon has a solid color mookaite border (frame) around the nipomo marcasite center. The marcasite polishes up similar to pyrite with a brassy yellow, metallic color. Just above the marcasite is some agate banding and a few crystal structures of quartz.
The second and third examples are framed with black jade. The marcasite appears to form in nice crystalline shapes that were later entrapped in the agate.