A peak with a distinct rocky summit, which dominates the landscape (539 m), approximately 1,5 km south of Bílina. The NNR was declared on a territory of 23, 24 ha in 1977. Bořeň is a massive exhumed laccolith of nepheline phonolite with steep erosional-denudational slopes. The subjects of the protection are the endangered plant communities with numerous specially protected species and the rare birds of prey which nest on the inaccessible rocks. Marked tourist trails and a view point are found on the peak.
The geological basement consists of two-mica and biotite cataslastic orthogneiss of Proterozoic age, which was covered by Creataceous – Turonian marlstones during the Mesozoic period. During the volcanic activity in the Tertiary Period the basement gneiss was lifted and broken and later in the Miocene a massive laccolith of nepheline phonolite penetrated the layers of Cretaceous marlstones. In the Quaternary period the resistant phonolite was exhumed from the surrounding soft marlstones and formed a unique and distinctive neovolcanic plug. This erosion was accelerated by the river flows (Bílina river and the predecessor of the present Ohře river) which carved a deep valley into the underlying gneiss to the west of Bořeň. The phonolite displays monumental columnar jointing with rocky walls up to 100 m high. Rock needles, pillars, columns and a fissure cave all developed as a result of frost weathering. The slopes are covered with stone and loam-dominated talus and several talus fields are found on Bořeň.
The steep slopes on the NW, N and NE expositions of Bořeň with high cliffs and talus are predominantly populated by acidophilous species such as the small-reed Calamagrostis arundinacea and by the critically endangered relict species Alpine aster (Aster alpinus). The stool iris Iris aphylla, the Cheddar pink (Dianthus gratianopolitanus) and the Irish saxifrage (Saxifraga rosacea) are highly endangered and the basket of gold (Aurinia saxatilis) is an endangered species. In contrast, the sunny S and SW slopes are the home to xerophilous and thermophilous grassland communities with the highly endangered Bohemian small pasque flower (Pulsatilla pratensis subsp. bohemica) and the endangered purple mullein (Verbascum phoeniceum). The remnants of meadows and pastures on the edge of the reserve are gradually becoming overgrown with shrubs and trees. Common species here include martagon lily (Lilium martagon), cowslip (Primula veris) and common twayblade (Listera ovata). Parts of the peak of Bořeň are forested with thermophilous oak and talus and ravine forest communities.
Ninety two bird species have been identified at Bořeň, of which twenty two are on the list of specially protected species. The peak has been a regular nesting site for eagle owls (Bubo bubo) for many years. Important vertebrate species also include the green toad (Bufo viridis) and the smooth snake (Coronella austriaca). Eighteen mollusc species have been identified on Bořeň, including xerophilous, but also forest species. The presence of 5 species of whorl snails (Vertigo pusilla, Vertigo antivertigo, Vertigo pygmaea, Vertigo alpestris) and the narrow-mouthed whorl snail (Vertigo angustior) is remarkable.
Forestry does not play a major role at Bořeň as the NNR is mostly rock covered.
A themed nature trail leads from Bílina to the viewpoint on the summit and rock climbing takes place on the NNR territory. Management of the forest growths on the edge of the reserve is aimed towards eliminating non-indigenous species such as Austrian pine (Pinus nigra). Clearance work is regularly carried out to slow the development of shrub vegetation on the talus slopes and xerophilous and thermophilous grassland areas. The locality is included in the Natura 2000 network for the protection of the vegetation of rocks, rocky slopes and scree.