Each iron oxide colour is not just a single oxide, but a mix of oxides or ferric oxides (rust) with manganese or coloured clay. Each colour tone can be altered by kiln temperatures, the concentration of minerals, pH, and the size of each particle.
Synthetic iron oxides are man-made but are actually made from natural ores and minerals, and they are widely used in the cosmetic field. They are considered to be non-toxic, moisture resistant, and colourfast.
Iron oxides graded safe for cosmetic use are produced synthetically to avoid the inclusion of ferrous or ferric oxides.
The synthetic iron oxides are used because they deliver light fastness and excellent dispersing properties into the final product and being intense in colour give great depth.