Updated: Nov 11, 2021
I love Turkey, and the country also has some interesting mineral fields.
When visiting a mineral shop, I fell in love with this combination of dendritic agate and opal, found in Eskişehir Province, Central Anatolia, Turkey.
When searching on the internet, I actually had some problems understanding what is Agate and Opal, as many sellers label everything as dendritic Opal.
After some hours of searching the net, I found some great explanations, all in Turkish. So, I have translated and hope this will help you understand. It is also more fun to owe a specimen when you understand more of where it comes from and how it occurs in nature.
Eskişehir and history
“Stones have been mined from the chalcedony deposits in Eskişehir for thousands of years. Chalcedony was extracted and used during the Hittite and Urartian periods. Later, during the Roman period, the export of the stone reached very high levels. During the Roman period, the stone began to be shipped for export from the port in today's Istanbul Kadıköy. At that time, Kadıköy was a small Ancient Greek town, and its name was Chalcedon. The chalcedony stone got its name from this small port town.
Eskişehir is also very rich in terms of Chalcedony, known as "ornamental stone" and "blue gold". There are plenty of ornamental stones such as chalcedony, milk opal, dendrite opal, chrysoprase, jasper, green opal, agate, and similar stones in Eskişehir and its surroundings, especially in the Sakarya Valley formed by the Sakarya River.”
Eskişehir: Dendritic Agate and Opal
The layer of the host rock is 5-25 meters and has a length of about 1,5 km. The host rocks mainly consist of well-rounded pebbles, opal, ultramafics, sandstones, limestones, and volcanics.
“The mainly white and black agate nodules range from 5 to 30 cm in diameter and show a zoned macrostructure. The transparent core is surrounded by a translucent white zone, which, in turn, is surrounded by a zone rich in black dendrites. An opal-rich zone is present nearest the outermost rim.
XRD analyses revealed alpha quartz (chalcedony), opal-CT (pseudocrystalline cristobalite), opal-C (pseudocrystalline tridymite), and moganite as the main mineral constituents of the agates. Under polarized light, the predominance of microcrystalline, fibrous quartz (“chalcedony”) is observed. The black color and fabric of the third, outer zone are due to the presence of black pyrolusite and manganite dendrites. SEM images also show different textures in agates that confirm the presence of a zoned structure in the dendritic agates.”
Before reading this, I had a little trouble understanding, and also because many of the nodules are cut when you buy a specimen. So many thanks to the people who wrote this.
The outer layer (host rock) is sandstone, inside that there are several nodules, like on this specimen:
The outer layer of the nodules would be opal. As far as I know, this opal would be milky white or translucent yellow.
Example of yellow translucent opal with dendrites. This is not a part of the nodule, but is located in the layer outside:
You will see tree-like black dendrites grow from the outer layer and into the middle, like in this video:
The core of the nodules is the Agate, which could be like pictures at the top and the video, but also translucent like the picture beneath:
The term "dendrite" comes from the Greek word dendron (δενδρον), which means "tree", and they are made of manganese/iron silicates.
You will see that some of the specimens are very rich in manganese/iron at the outer layer of the nodules, making a black outer layer (like seen in the video above).