These geodes are the reason aura coating exists.
The story told in the trade is that they were first mined in the 1970s by the owner of Rancho Agua Blanca and brought north into the United States, where a dealer in El Paso began electroplating them with metal vapour to give them an iridescent film. They sold. Every jewellery house wanted them, the deposit was worked hard, and the technique spread to everything else on the shelf. The rainbow coatings you now see on quartz points started here, on geodes smaller than a grape.
That history is why the treatment line on this listing matters more than usual. Coated and dyed geodes are still everywhere, and a metal film is very difficult to spot once it is on.
This one has nothing on it. What you are looking at is the colour the silica came out of the ground with.
Mineral: Quartz (silicon dioxide, SiO₂), a chalcedony rind with druzy quartz inside
Crystal system: Trigonal
Origin: Rancho Agua Blanca, Tabasco Municipality, Zacatecas, Mexico
Hardness: 7 on the Mohs scale; hard, though the rind is thin and will crack if dropped
Treatment: None
Dimensions: 2.08 x 0.74 x 3.08 cm
Weight: 13.09 g
You will receive the exact piece photographed.