Agates form in pockets known as nodules in igneous rocks of volcanic origin, these mineral specimens show the round nodule, almost pebble like, very well. Agate is a variety of chalcedony and forms when silca is deposited within pockets of gas in an igneous body, these deposits can form in layers, giving a banded appearance when the samples are split. Depending on the chemical nature, impurities and any intrusions in to the pocket, other minerals such as quarts crystals can also form inside as well. Thus creating beautiful, eye catching patterns and bands of alternating colours. These particular specimens come from Perthshire in Scotland and are fairly rare, they are available as single pieces or matching pairs and come supplied in a card tray with an information card.