Agate was a useful mineral to early man as well as Native Americans since Agate can be chipped and fractured in any direction, making  it a perfect material to shape tools and weapon points.

These Agate arrowheads have been anodized. This process creates an oxide layer on the outside of the mineral, which absorbs, refracts and reflects certain wavelengths of light producing the observed colors.  The color depends on the thickness of the oxide layer.