Amethyst is a purple variety of quartz. Its regal colouring appears in a range of hues, varying from light lilac to deep indigo depending on the abundance of iron in its chemical composition. It has a vitreous lustre and is translucent.
Amethyst often grows along the inner walls of hollow igneous rocks, known as vesicles. Vesicles are produced by bubbles of gas in magma or lava creating cavities in the rock when solidified. Small cracks in these rocks allow for liquid silica to seep in, which then precipitates crystals of amethyst.
During the 17th century, the time when this frame was made, amethyst was found in a multitude of European sources, particularly in Bohemia and the Black Forest. It then would have been sold to the famous lapidary workshops of Idar-Oberstein in Germany, where the stones were cut, polished and then traded on to artisans and jewellers.
Nowadays, the largest commercial sources of amethyst are Brazil, Uruguay, Siberia and North America. The largest amethyst geode in the world is The Empress of Uruguay, which reaches 3.27 metres in height and weighs over 2.5 tonnes.