Opal is an amorphous substance, hydrated silicon dioxide (SiO2.nH2O), with a variable amount of water (3-21%). There are several opal varieties nonetheless the precious opal represents one of the most valuable ones thanks to opalescence, a magic light reflection of sparkling colourful pastel colours. Colour changing is caused by a refraction and dispersion of light on fine layers with different content of water, minute admixtures of other minerals or in micro-fissures. The precious opal is the most famous gemstone of Slovakia. It can be found in fillings of fissures in Late Tertiary volcanic rocks – andesites in areas between Červenica and Dubník in Slanské vrchy Hills. It was formed by condensation of hot silicate aqueous solutions which as after-effects of volcanic activity penetrated into fissures of neighbouring rock.
Precious opals from Dubník have already been known in ancient times. That time similarly as in the Middle Ages, it represented the only known opal field in the world.  It was believed that opals promoted happiness to their owner. It is said that Nonius, Roman senator, chose rather to be banished than to hand over his beautiful opal with a fine engraving to triumvir, Marcus Antonius, Cleopatra’s lover. Also members of the family of Emperor Napoleon took fancy to opal from Dubník, Empress Josephine used to wear gem opal called “The Fire of Troy”. The biggest Slovak opal is “Harlequin” and was found in a brook in 1775. It has a form of a thin wedge with dimensions of 12.5 × 5.7 cm and variable thickness of 2-7 cm. Its weight is 600 grams and is stored in collections of the Museum of Natural History in Vienna. However, the fame of opal from Dubník started to decline after the discovery of rich deposits in Mexico and mainly in Australia in 1875. Nowadays, more than 95% of gem opals come from this deposit. Opal mines of Dubník were closed due to their unprofitability in 1922, at present they represent an area protected by the state as a significant wintering site for bats.
On the gutter block is located quartz – agate from the area of Kuzmice located in the southern part of Slanské vrchy Hills in eastern Slovakia. A crystal filling of fissure in basalt – aragonite from the area of Bulhary near Fiľakovo in southern Slovakia is depicted on FDC. And a crystalline structure of libethenite – secondary phosphate of copper from Ľubietová, which is one of the 19 Slovak minerals first found and noted down in Slovakia, represents a motif of the FDC cancellation.

                                                                                                                                                                                                               Ján Madarás and Milan Gargulák