Amber is amorphous, of resinous lustre and usually found in small pieces: irregular lumps, grains, drops and stalactites. It feels warm, is lightweight and porous and may fluoresce naturally under daylight, especially when freshly extracted. Inclusions of organic matter—insects, crustacea (some now extinct), flora, bark etc—resulted from these being trapped in the liquid resin as it flowed downwards. When in contact with atmospheric air, its surface becomes oxidized and forms a crust. Transparent, opaque (due to an abundance of tiny bubbles) or osseous, it is commonly yellow to honey-coloured, but approximately 250 different colour varieties including white and black are known, the rarest being red, blue and green. On lengthy exposure to air, golden-yellow amber slowly darkens to red. Green amber is thought to have formed in marshy areas through inclusions of decaying organic material.