An '''opal''' is a mineraloid composed mostly of silica and water. Opals can express every colour in the visible spectrum, although the most common colours are white and/or green.

While tidying up the cabinets next to the mantelpiece at [[12 Grimmauld Place]] in [[1995]], [[Harry Potter]] came across a large blood-filled crystal bottle with a large opal set into the stopper.<ref name="OP6">''[[Harry Potter and the Order of the Phoenix]]'' - Chapter 6 (''The Noble and Most Ancient House of Black'')</ref> There was a [[curse]]d [[opal necklace]] for sale at [[Borgin and Burkes]] that [[Draco Malfoy]] purchased and gave to and [[Imperius Curse|Imperiused]] [[Katie Bell]] in an attempt to assassinate [[Albus Dumbledore]].