8: Chalcedonies

Different types of chalcedonies

Different chalcedonies have hence been called by their own names for a long time. The most common ones are agate and jasper. Agates are usually translucent at least to an extent, and even the naked eye can distinguish the different-coloured, generally circular bands. Jaspers can also be banded, but they are also opaque, allowing barely any light to pass through them. Sometimes the name chalcedony is used to refer to an even blue or blue-and-purple variant of chalcedony. Different chalcedonies occur all over the world, and many of them are rather commonplace. On the other hand, several types of chalcedonies are rare. The most valuable of them is the bright green chrysoprase. Agates and other chalcedonies have been found in Finland as well; for instance, in the Sääksjärvi, Huittinen and Lappajärvi impact craters. Finnish Lapland also has aventurine deposits.

Agate

Agates can be found almost in the full spectrum of colours. Agates are also sometimes dyed. The rarest colour in natural agates is purple. Agate is a hard stone type (Mohs 6.5–7). It can be cut and polished into thin, smooth slices. This makes the patterns and colours more easily distinguishable. Agate is therefore well suited for use in jewellery and ornaments.

Large agate deposits are known especially in the westernmost United States. The majority of agate is now mined in Brazil. Countless different types of agates have been named and described based on their appearance. The best-known ones are the pale blue and white-patterned lace agate, moss agate exhibiting green moss-like patterns, landscape agate with patterns resembling whole natural landscapes, and the bright reddish-orange fire agate.

Jasper

Jasper deposits can be found all over the world, including Northern Finland. There are many different jasper varieties, but many stones sold as jasper are not actually jasper; for instance, the leopard skin jasper found in Mexico is actually rhyolite, and the Kambaba jasper found in Madagascar is a volcanic stone consisting of feldspar, quartz, aegirine and amphibole.

Carnelian

The name carnelian comes from the Latin word cornus, meaning the cornel cherry (Cornus mas). The colour of the red fruits of this Mediterranean shrub is similar to that of carnelian. The first carnelians were found in India. Later they were discovered in Germany, Russia, the United States, Peru, Thailand and Japan, among other places.

Chrysoprase

According to Albertus Magnus, a German bishop and philosopher from the 13th century, the Macedonian king Alexander the Great (approximately 356–323 BCE) always wore a chrysoprase amulet during battles, believing it would bring him good luck. Chrysoprase has been mined in Germany, Poland, the United States and especially Australia, where they have also discovered the world’s largest chrysoprase, weighing 16.4 tonnes.

Onyx

Black agate can look uncannily similar to onyx, but the surface of black agate has a glass-like lustre, and the bands are unstructured, often circular. Onyx has been used a great deal in the production of jewellery and ornaments ever since the Roman Empire. According to legend, the Egyptian queen Cleopatra (51–30 BCE) carried onyx with her as a form of protection. Onyx occurs in Greece, the United States, China and around the world in other regions with volcanic activity, where it forms slowly in cavities when water evaporates and the rock solidifies.

Aventurine

If the inclusions are green fuchsite, as they generally are, the entire aventurine turns a beautiful green colour. It is in fact the best-known and desired colour of aventurine. Sometimes the inclusions can be hematite or goethite, which produce red or orange aventurine. Blue and grey aventurines have also been discovered. Like many green stones, green aventurine has been used in amulets to bring good luck. The most famous aventurine mines reside in India. The stone has also been found in the United States, Russia and Japan, but also in Finland, where it has been found at least in Lapland.

Conchas

Conchas are thought to have been formed by slowly solidifying in the cavities of volcanic rock material. Southern parts of the United States as well as Mexico are important sources.