Foitite Identification Guide
How to identify foitite, a rare iron-rich blue-black tourmaline, by its trigonal prisms, hardness, lack of cleavage, and the tests that separate it from schorl and other tourmalines.
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What Foitite Looks Like
Foitite is a rare, X-site-vacant, iron-rich member of the tourmaline group (an alkali-deficient cousin of schorl). It is a complex borosilicate and is mostly of mineralogical and collector interest rather than a gem.
- Color: dark bluish-black to blue-grey, sometimes with a violet or indigo cast in thin splinters — its bluish tone helps separate it from jet-black schorl.
- Luster: vitreous.
- Transparency: translucent on thin edges to opaque.
- Crystal habit: slender to acicular trigonal prisms with the characteristic rounded-triangular cross-section and prominent length-wise striations; often in radiating sprays or as fine needles in quartz.
Step-by-Step Field-ID Checklist
- Confirm tourmaline first. Look for elongate prisms with a rounded triangular cross-section and strong vertical striations — the universal tourmaline tell.
- Check color in thin section/edges: a bluish or indigo cast rather than pure black hints at foitite over schorl.
- Test hardness against glass.
- Look for NO cleavage — tourmalines fracture, they do not cleave.
- Note the setting: often in granitic pegmatites or hydrothermal quartz veins, sometimes as needle inclusions.
Key Diagnostic Tests
- Mohs hardness: ~7–7.5. Scratches glass readily.
- Streak: white to pale grey/bluish.
- Cleavage: none/indistinct; uneven to conchoidal fracture — this rules out amphiboles and micas.
- Density: ~3.1–3.2 g/cm3.
- Non-magnetic, no acid reaction.
- Definitive ID requires lab analysis: because foitite is chemically defined by an alkali-deficient X-site, only EDS/microprobe chemistry (or careful optical work) reliably separates it from schorl. Hand specimens are usually a tentative ID.
Common Look-Alikes and How to Tell Them Apart
- Schorl (black tourmaline): the closest twin. Schorl is sodium-bearing and jet-black; foitite is alkali-deficient and tends bluish. They are visually almost identical — chemical analysis is the only certain separator. A bluish tint in thin splinters favors foitite.
- Dravite/elbaite (other tourmalines): different colors (brown, pink, green); same crystal form, so use color plus chemistry.
- Hornblende/other amphiboles: dark and prismatic, but amphiboles have two directions of cleavage at ~120/60 degrees, while foitite has none. Cleavage is the quick separator.
- Black quartz (morion): hexagonal prisms with no striations like tourmaline's, conchoidal fracture, and no triangular cross-section.
- Augite/pyroxene: has cleavage near 90 degrees and stubbier crystals.
The practical field call is tourmaline form (striated trigonal prism, no cleavage, hardness ~7) + bluish-black color, with the caveat that confirming foitite specifically needs lab chemistry.
Where Foitite Is Found
Foitite was first described from San Diego County, California (USA). It occurs in granitic pegmatites and hydrothermal quartz veins, and has been reported from localities including the USA, the Czech Republic/Elba (Europe), and other classic tourmaline pegmatite regions, typically as a late-stage, alkali-poor tourmaline.
Frequently asked questions
How can you tell if a stone is foitite?
First confirm it is tourmaline: an elongate prism with a rounded triangular cross-section, strong lengthwise striations, hardness about 7–7.5, and no cleavage. A bluish-black rather than pure-black color hints at foitite, but because foitite is defined by its alkali-deficient chemistry, certain identification requires laboratory analysis to separate it from schorl.
What is the difference between foitite and schorl?
Both are dark, iron-rich tourmalines that look almost identical. Schorl is sodium-bearing and jet-black, while foitite is alkali-deficient (X-site vacant) and tends toward a bluish-black. Visually they are nearly the same, so chemical analysis is the only reliable way to tell them apart.
Is foitite a gemstone?
Foitite is rarely cut as a gem. It is a dark, mostly opaque, rare tourmaline of primary interest to mineral collectors and scientists rather than the jewelry trade.
How do I tell foitite from hornblende?
Foitite is tourmaline and has no cleavage, fracturing unevenly, with striated triangular prisms and hardness about 7. Hornblende is an amphibole with two good cleavages meeting at about 120 degrees and is softer (5–6). The cleavage and hardness tests separate them.