Ruby is exposed to heat treatment in order to enhance the colour. This is considered to be a stable and permanent enhancement. Intact rutile inclusions (also known as "silk") provide proof that a ruby gemstone has not been heat treated. Ruby is also irradiated and treated with diffusion to enhance the colour. Other enhancements include fracture filling with lead glass, which is conducted to render heavily included gemstones fit for jewellery use. Occasionally, ruby is oiled or dyed. Despite all of the treatments that ruby can be subjected to, natural ruby gemstones are available and all reputable gemstone sellers declare any treatments or enhancements.