Natrolite is a common mineral from the zeolite group, known for its needle-like crystals and striking fluorescence under UV light. Chemically, natrolite is a sodium aluminum silicate with the formula Na₂Al₂Si₃O₁₀-2H₂O. This mineral is prized for its interesting crystal structure and ability to fluoresce in different colors under UV light, with the lemon yellow fluorescence being particularly striking. The lemon-yellow fluorescence observed in natrolite from Soutěsky is particularly striking and is often caused by the presence of small amounts of activator ions such as iron or other metal ions. These ions can trigger electron transitions in the crystal lattice, leading to the emission of light in the yellow spectral range.

Soutěsky, a small town near Děčín in the northern Czech Republic, is known for its significant deposits of natrolite. The geological conditions in this region have led to the formation of natrolite crystals, which stand out due to their clear transparency and impressive fluorescence under UV light. This region is particularly famous for natrolite samples that show a strong lemon-yellow fluorescence when exposed to UV light, a characteristic that distinguishes them from natrolite from other locations.