Opal can be subjected to various types of treatment. Present CIBJO guidelines state that any method of treatment other than standard cutting and polishing must be disclosed and the process used specified on all invoices, advertising, and commercial documents. Types of treatments include colour and/or tone enhancement, heating, painting, dying, resins and waxes, oiling or any application of chemicals. Opal also may be treated to change its natural appearance or durability. (See the discussion on composite opals, below). The body tone of opal is often enhanced (usually appearing darker) in some opal inlay jewellery where a thin piece of solid crystal opal has black paint or black glue applied, or the opal is set above black painted jewellery.