Crystals in the veins of an oak (Quercus sp.) leaf, polarised light micrograph. The crystals found in the veins of oak leaves are typically calcium oxalate crystals, which are common in many plant species. These crystals form as a result of metabolic processes in the plant and are often located in specialised cells called idioblasts, which are present in the veins and other tissues of the leaf. Calcium oxalate crystals in oak leaves can take on various shapes and sizes, including needle-like or rhomboid crystals. These crystals are formed from the precipitation of calcium oxalate, which is produced by the plant's metabolism. The formation of calcium oxalate is thought to be a way for plants to regulate calcium levels and protect against herbivory and other environmental stressors. Calcium oxalate crystals are not harmful to humans, but they can cause irritation if ingested or come into contact with the skin or eyes. In oak trees, calcium oxalate crystals may also form in other parts of the plant, such as in bark, roots, and acorns. These crystals can be used to identify different oak species and may have various ecological functions, such as deterring herbivores or contributing to nutrient cycling in the soil.