The so called Janos-Agates ( sometimes called “Rainbow-Agate”) are the formatized, trimmed and knapped centers of big thundereggs from Ejido Ojo Caliente in the area of Cerro el Tascate near the village Janos. These really big Thundereggs (they are reaching diameters of 30 centimeters and more ... ) are presenting only small and thin agate veins (up to 30 mm wide). But these agate-veins are intensively coloured in yellow, green, blue, violett and all kinds of reds. The formation of big colour-pigments seems to be distinctive for the Janos-agate.
More informations about the locality and the agate-formation is given by Brad L. Cross in his book “The Agates of Northern Mexico” and by J. Zenz in his book “Agates”, Bode Verlag 2005, pages 442 and 443 .