Turquoise is a blue, blue-green or grey-green opaque gemstone typically found in dry, and arid regions where acidic, copper-rich groundwater percolates the stone and reacts with phosphorus and aluminium, which results in the formation of turquoise nodules and veins. Copper particles give turquoise its distinctive blue colour; greenish hues are usually caused by either iron pollutants or dehydration. Depending on the rock in which turquoise develops, it may be interspersed with a matrix of spidery veins – dark and brown for limonite, or beige for sandstone. Occasionally, one can find turquoise speckled with bits of pyrite. Turquoise has a rather porous structure due to its polycrystalline nature. Smoother and sleeker stones are more valuable; their durability is also higher.