Diaboleite is a blue-colored mineral with formula Pb₂CuCl₂(OH)₄. It was discovered in England in 1923 and named diaboleite, from the Greek word διά and boleite, meaning "distinct from boleite".^[3] The mineral has since been found in a number of countries.

Diaboleite is deep blue in color and pale blue in transmitted light. The mineral occurs as tabular crystals up to 2 cm (0.8 in) in size, as subparallel aggregates, or it has massive habit. Vicinal forms of the tabular crystals have a square or octagonal outline and rarely exhibit pyramidal hemihedralism.^[2]

Diaboleite occurs in manganese oxide ores, as a secondary mineral in lead and copper oxide ores, and in seawater-exposed slag. Diaboleite has been found in association with atacamite, boleite, caledonite, cerussite, chloroxiphite, hydrocerussite, leadhillite, mendipite, paratacamite, phosgenite, and wherryite.^[2]

A study in 1986 synthesized diaboleite crystals up to 0.18 mm (0.0071 in) in size using two different methods. The study demonstrated that diaboleite is a low-temperature phase, that is stable under hydrothermal conditions at temperatures less than 100 to 170 °C (212 to 338 °F). At higher temperatures, the first stable mineral to form is cumengeite.^[4]

In 1923, diaboleite was discovered at Higher Pitts Mine in the Mendip Hills of Somerset, England,^[3] and described by L. J. Spencer and E.D. Mountain.^[5] The study of the similar mineral boleite was perplexing at the time and this new mineral only compounded the difficulty. As insufficient material was available for a full investigation, Spencer and Mountain named it diaboleite, meaning "distinct from boleite", out of "desperation".^[6]

The mineral was grandfathered as a valid mineral by the International Mineralogical Association as it was described prior to 1959.^[3]

As of 2012^[update], diaboleite has been found in Australia, Austria, Chile, France, Germany, Greece, Iran, Italy, Russia, South Africa, the UK and the US.^[2][3] The type material is held at the Natural History Museum in London and the National Museum of Natural History in Washington, D.C.^[2]