Two halves of one geode, cut and mounted facing each other as a butterfly.
The pair was a single hollow cavity until it was opened. Everything across the two faces grew as one continuous lining, which is why the banding on the rind runs from one side to the other and the crystal field carries on across the gap. Cut once, and there is no second pair.
The cavity itself formed as a gas bubble in cooling lava. Silica-rich water later worked its way in and crystallised inwards, quartz first, then amethyst as iron entered the system. The purple is iron held in the quartz structure and altered by natural radiation in the ground over a very long period.
Cut geodes are reinforced before they leave the mine. The outer shell on a piece this size can be close to eggshell-thin, so the back, base and sides are cemented and the exterior painted dark to match the basalt it came out of. That is done to every cathedral and butterfly on the market and it is almost never mentioned. Nothing has been done to the crystal itself.
Keep it out of direct sunlight. Sustained UV breaks down the iron centres that produce the colour, and amethyst that fades does not come back.
Mineral: Amethyst, the purple variety of quartz (SiO₂)
Crystal system: Trigonal
Origin: Brazil
Hardness: 7 on the Mohs scale; hard, though individual crystal tips chip easily
Treatment: None to the amethyst. Backing, base and exterior are cemented and painted, standard practice for cut geodes
Dimensions: 36 x 17 x 35.5 cm
Weight: 9.1 kg
You will receive the exact piece photographed.