Agate geode pair – #6

Agate is a popular mineral that is often found in volcanic cavities. The often layered stone is a crypto-crystalline form of quartz, which means that the crystals form on a microscopic scale and are not detectable by the naked eye. The wide range of colours in which agate is found is caused by metal ions dispersed throughout the mineral. Agate forms when in water dissolved silica (quartz) flows past a cavity in the rock while the liquid, also called silicic acid, is cooling down. Because of the cooling, the water can contain less quartz and it deposits these quartz-molecules. Depending on the circumstances, the quartz can form in macro- or microcrystalline (also called cryptocrystalline) shape. In the last case, Agate can come into existence. The name Agate is derived from an ancient name for the river Dirillo, namely Achates, in Sicily, where Agate was first discovered.