The Slovak post Pofis issued a set of two stamps within the series "Nature Protection" depicting Slovak minerals – precious opal from Dubnik and scepter quartz from Sobov.
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 Cervenica and Dubnik in Slanskevrchy Hills. It was formed by condensation of hot silicate aqueous solutions which as after-effects of volcanic activity penetrated into fissures of neighbouring rock.
Quartz (SiO2) is one of the most common minerals in nature forming hexagonal columnar crystals finished with planes of trapezohedron or dipyramid. Crystal planes are frequently grooved. The crystals can be found in a form of crystal twinning and other various deformations. However, this mineral usually has massive, granular or microcrystalline shape, and can be also found as amorphous. It can be of various colours, virtually covers the whole colour spectrum. Its streak is white. Quartz is a transparent even translucent mineral with vitreous shine on a new surface. Plenty of its forms represent semiprecious stones, for instance: clear crystal quartz, rose quartz, violet amethyst, yellow citrine, smoky quartz, black morion, amorphous chalcedon variable in colour, agate and opal.
A so-called sceptre or dovecote quartz is a mineralogical curiosity from the vicinity of BanskaStiavnica. The crystals are joined together along the longest axis in such a way that their final shape reminds a sceptre or a dovecote (the bigger crystal is usually grown on the smaller one or vice-verse).