Agates are only a by-product at Las Choyas. Mining is done for geodes. There is a per year production of over 75000 pounds of nodules. Las Choyas near Cerro Mesteno is about 22 miles east-northeast of Ojo Laguna. Geodes and agates from that location are also known as"coconuts".
They appear in an altered ash-flow tuff. The unit is known as Liebres Formation and is about 44 million years old.Banding is mostly blue-grey, shadow agates are not rare. The size of the nodules is between 1,5 to 8 inches.
Further informations are given from Brad L. Cross in his book “The Agates of Northern Mexico”, page 99-102