Motifs: The Agate of Lepoglava and Limestone from Brač - from the funds of the Mineral and petrographic collection of the Croatian Museum of Natural History The science engaged in researching the composition of the Earth, its origin and the classification of rocks is called petrology while the minerals forming these rocks are subject of mineralogy. In order to be able to talk about rocks and minerals we first have to say something about the aspect and composition of our planet Earth. The Earth is composed of layers: first 50 km makes Earth s crust followed by the mantle till the depth of about 2900 km and the core reaching to the very centre of the Earth (6370 km). The stony crust and the litosphere make the solid part of the Earth which builds rocks. Rocks are aggregates of one or more kinds of minerals, while minerals are natural ingredients whose structure can be expressed by a chemical formula. Minerals have a regular inner structure and a regular arrangement of atoms which can influence the outer form of the mineral (such regular forms of minerals are called crystals). If a rock is built of only one mineral we call it a monomineral while the rocks that contain more minerals are called polymineral rocks. According to their origin rocks can be magmatic, sedimentary or metamorphic. About 95 percent of the lithosphere is made of magmatic rocks, while other two rock types build only five percent of the lithosphere. According to their chemical composition, minerals (today there are more than 4700 minerals known) are classified into elements, sulphides, halogenides, oxides and hydroxides, carbonates and nitrates, borates, sulphates, phosphates and a large group of silicates which make more than 95 % of the composition of the Earth s crust. The Agate of Lepoglava One of the most common minerals in the Earth s crust is quartz (SiO2). Banded microcrystalline quartz (chalcedony) is called agate, the name deriving from the Greek word for a river in Sicily (Achates). It comes in rounded, stalactite-like or irregular forms that in cross-section have zonal structure. Its hardness is somewhat smaller than the hardness of quartz, but it is more resilient and therefore apt for treatment, and since it is also chemically very stable, it is also used in the production of laboratory utensils and appliances. The coloured varieties can often be transformed into jewellery. Though agate can develop in several ways, it most often appears together with volcanic rocks. This is also how the agate of Lepoglava developed, together with lavas pouring some twenty million years ago along marine fissures in Miocene sea, and later appeared on surface as solid rocks due to tectonic movements. The agate of Lepoglava has the shades of light blue, greyish-blue to grey colour. Today, apart from at Kameni vrh it is secondarily found in ditches and alluvial deposits in the wider area of Lepoglava. By the Nature Protection Act this finding site of agate was in 1986 proclaimed geological nature monument. In 2006 the society The Agate of Lepoglava(Lepoglavski ahat) was founded which promotes the values of the Lepoglava region not only as the finding site of agate but in general as an interesting mineralogical and geological phenomenon. Through the association ProGeo Croatia, the society The Agate of Lepoglava is included into the International Union of Geological Sciences. The purpose of the society is acquiring knowledge on geological particularities of the Lepoglava region and the transmission of this knowledge to wider social community.
