NEWS
04 JUNE, 2013
What is Czech garnet?
It is a precious stone from a group of 17 garnets, most of them very rare. Two groups are more commonly found in nature, i.e. aluminium and calcic garnets.
There are three aluminium garnets: Czech garnet, almandine, and spessartine. It is coloured by iron and chromium into a fiery or dark red, the colour saturation remaining the same even in the smallest grains of around 2 mm. It perfectly combines with almandine, and even today the two are not easy for scientists to tell apart. Its light refraction is measured by an apparatus called the refractometer; its so-called refractive index is around 1.75. Density (specific gravity) as measured by immersion in a liquid and weighing in air should be 3.8. When the index is higher, the mineral is called rhodolite or pyralmandine. It can be called almandine only after it has reached a refractive index of 1.79 and a specific gravity of 3.95. It is non-combustible: it does not turn grey when heated but after cooling down returns to its original colour. Czech garnet can be sealed in glass, clay, or enamel. Most other precious stones lose colour and crack after heating. Under the microscope, magnified 5 to 70 times, Czech garnet is perfectly clean or has only minor black blemishes. Almandine has on the inside a dense network of needles of another mineral -- rutile. In prehistoric and historic times, the only European sites with Czech garnet were in Bohemia, mainly in the České Středohoří but also in Podkrkonoší and around Kolín nad Labem.