Red in agate is usually caused by iron oxides. The dinosaur bone is a replacement by agate, colored, in part, by iron oxides in the agate material. Fire agate is even more directly affected by iron, as it causes all the iridescent colors.
One of the most popular agates for collectors is Lake Superior agate, which is usually red with clear, white, brown, and/or black banding. Sometime it’s gray, and other colors and patterns occur.
Header Photo: Drusy cabochons from Brazilian Agate
Top Left Photo: Dinosaur Agatised Bone see link below
Middle Photo: Fire Agate see link below
Bottom: Lake Superior Banded Agate photo by Clara Darlene Reger