Amblygonite gems are usually pale straw yellow. Although they are too soft and cleavable to make good ring stones, collectors prize them if they show darker colors. Large faceted stones are extremely rare.
(Li, Na) Al (PO4)(F, OH). Usually Li greatly exceeds Na.
Amblygonite comes from the Greek amblus for “blunt” and gonia for “angle,” alluding to the shapes of its crystals. Montebrasite is named after its type locality, the French town of Montebras.
Commonly, veil-type inclusions, usually clouds in parallel bands.
Amblygonite and montebrasite form a mineral series. Fluorine-dominant (F) amblygonites are much rarer than hydroxyl-dominant (OH) montebrasites. In addition, amblygonite has a biaxial (-) optic sign, while montebrasite has a biaxial (+) optic sign. Many gems labelled as amblygonites in collections may merit reexamination.
Most yellow gems in this series, found in collections and on the market, are amblygonites from Brazil. However, stones from Mogi dãs Cruzes, Sao Paulo, Brazil, are montebrasite.
Extremely rare material from Karibib, Namibia shows a lilac color. These stones also make beautiful faceted gems.
Note that refractive indices and optic angles decrease as sodium (Na) and F content increase. The change in optic sign (where 2V = 90°) occurs at around 60% (OH). A complete series of (OH, F) substitutions appears to exist.
You’ll more likely encounter these stones in mineral collections than jewelry collections. However, if you want jewelry made from these gems, use protective settings and avoid ring use. Pendants, brooches, and earrings shouldn’t pose too many risks. Also, store this jewelry separately from more common, harder gem materials, like quartz, to avoid contact scratching.