A @[email protected] in which the @[email protected] is composed of ionic species supported in a crystalline host. The first @[email protected] used a ruby crystal in which chromium ions as the @[email protected] were supported in an aluminium oxide (sapphire) crystal. One of the most common crystal lasers involves neodymium atoms in a yttrium aluminium garnet host crystal (the @[email protected]:Nd @[email protected]).