Amblygonite: Gemstone Information
Amblygonite (Li,Na)Al(PO4)(F,OH) forms a series with montebrasite in which the hydroxyl component exceeds that of the fluorine. Almost all faceted stones are montebrasite. They are an attractive, bright yellow green, crystallizing in the triclinic system in which they form equant to short-prismatic crystals with twinning common. There is one direction of perfect and one of good cleavage; the hardness is 5.5–6 and the SG 3.00–3.10 (values for amblygonite-montebrasite is 2.98). The RI of amblygonite ranges from 1.578 to 1.591 for the alpha ray and from 1.598 to 1.612 for the gamma ray. The corresponding values for montebrasite are 1.594–1.611 and 1.616–1.633. The optic sign is positive for montebrasite and negative for amblygonite. Some variable but unimportant luminescence may be seen. Both minerals are characteristic of granite pegmatites, the clear Brazilian lemon-yellow montebrasite occurring in particular from Linopolis, Galileia; also from Aracuai, Governador Valadares and Itinga, Minas Gerais. Gemstones from Mogi das Cruzes, Sao Paulo are reported to be amblygonite rather than montebrasite.