The Křížky NNM was declared in 1962 and covers an area of 4 hectares at elevations between 788 and 817 metres above sea level. The monument lies around 1.5km north of the village of Prameny in the Karlovarský kraj - Karlovy Vary Region. The protected area consists of a deforested serpentinite rock with three crosses on the summit and is the home of rare plant and animal species in the Slavkovský les Forests (Protected Landscape Area), and especially the endemic sandwort-leaved mouse-ear (Cerastium alsinifolium). Access to the locality is restricted to help protect these rare and endemic species. Visitors can learn about the unique nature found in the Křížky National Nature Monument and the neighbouring Upolínová louka pod Křížky NNM from the series of information panels which have been installed here.
The geological structure of Křížky NNM is very simple as it covers a part of a morphologically distinctive ridge, which is built of bronzite serpentinite. Serpentinites are a greenish grey-black massive rock, with small phenocrysts of metamorphosed pyroxene, narrow veins of actinolitic asbestos and phenocrysts of talc scales. The predominant mineral components are fine-grained antigorite, ore segments of magnetite and chromite, chlorite, accessory spinel, rutile with relict bronzites and tremolites in places. Brown-black opal formed here by a process of secondary metamorphosis. Several other smaller outcrops of serpentinite can also be found in the vicinity of Křížky NNM.
When the serpentinite rock breaks down it forms a selective substrate with a specific chemical composition, which certain plant species are bound to. The most exceptional plant species to grow here is the very rare sandwort-leaved mouse-ear (Cerastium alsinifolium), which is an endemic species in this region, that grows nowhere else in the world except the serpentinite outcrops in the Slavkovský les Forest, as well as the critically endangered Sudetic woodruff (Galium sudeticum). Other notable species which grow at Křížky are especially the serpentine species of ferns which can be found in rocky fissures, on rocky terraces and talus: ladder spleenwort (Asplenium adulterinum) and black serpentine spleenwort (Asplenium cuneifolium). We can also find the forked spleenwort (Asplenium septentrionale), brittle bladder fern (Cystopteris fragilis) as well as the tiny moonwort (Botrychium lunare) on the heathlands.
In the spring months the rocks are decorated with numerous pink flowers of the spring heath (Erica carnea) and heather (Calluna vulgaris) which flowers later in the year. The heathlands and short-stemmed grasslands are a suitable biotope for the shrubby milkwort (Polygala chamaebuxus), the medicinal mountain arnica (Arnica montana), mountain everlasting (Antennaria dioica), lousewort (Pedicularis sylvatica), broad-leaved marsh orchid (Dactylorhiza majalis) and the lesser butterfly orchid (Platanthera bifolia).
The deforested serpentinite rocks and their surroundings are the home of a range of rare and protected animals, especially invertebrates. These include the locust Psophus stridulus and the swallowtail butterfly (Papilio machaon). The moorland clouded yellow (Colias palaeno) lives on the growths of bog whortleberry at the monument. Reptiles which are represented here are the adder (Vipera berus), the smaller smooth snake (Coronella austriaca), slowworm (Anguis fragilis) and the sand lizard (Lacerta agilis). Black grouse (Tetrao tetrix) and quail (Coturnix coturnix) are among the interesting bird species which have been observed at the locality.