The formation of amethysts took place over the course of millions of years within the bedrock in the Crust layer as our planet endured drastic internal changes in pressure due to the flowing, cooling, and solidification of magma. Amethysts formed from a combination of elements, the first one being silica-rich minerals with a touch of ferric iron. Also, a suitable surface, radiation, and the right temperature were intimately involved. As pressure pushed these chemicals upwards they came across oxygen which turned the mix into silicon dioxide, this is the chemical base for quartz which hosts the purple amethysts.

In boiling magma, and within igneous rocks called basalt, pockets of gas formed. These blisters later solidified creating hollow bubbles which explored upwards. These sacs became igneous geodes, formed as the flowing magma transformed into flowing lava. Bedrock rich in different minerals was paramount, as well as the presence of cooling mineral-abundant water to seep through tiny cavities in the geode walls. When every factor was present in this intricate process, it all resulted in the formation of crystals sprouting into peaks from the geode’s outside layer inwards towards the geode's hollow center. 

Sedimentary geodes, scientists believe, formed and hardened around a host such as a tree trunk, or a shell of any kind. As the carbonaceous, organic materials decayed away, the cavity was filled with silica-rich fluids giving birth to the amethyst crystals.
When all these elements mixed in harmony throughout very scarce areas of the planet, some geodes evolved into astonishing formations of fossilized minerals, brilliant crystals called amethysts. These remarkable occurrences took place in the farthest depths of our Earth during the Cretaceous Inferior Period.