Fluorite, chemically known as calcium fluoride (CaF2), is a mineral highly valued for its striking coloration and distinct crystal structure. Due to its popularity in the lapidary and collector markets, imitation fluorite is common, often manufactured from materials like glass, plastic, or sometimes other minerals. Understanding its physical and optical properties is necessary to distinguish it from a counterfeit. Identification relies on a series of simple, observable, and measurable tests.
Visual Characteristics and Crystal Shape
Fluorite is often called “the most colorful mineral in the world” because it occurs in virtually every shade, including purple, blue, green, yellow, and clear. The colors are caused by trace impurities and natural radiation exposure, which means a single specimen might display color zoning or banding where different hues layer together. When light catches the surface, the mineral exhibits a vitreous, or glass-like, luster, and the material itself can range from transparent to translucent.
The most telling visual clue lies in the mineral’s crystal habit, which refers to its typical external shape. Fluorite crystallizes in the isometric system, most commonly forming perfect cubes, though it is often found as octahedrons, which are eight-sided, double-pyramid shapes. The characteristic cubic and octahedral forms are a strong initial indicator of fluorite, but they do not confirm identity alone.
The Hardness and Cleavage Test
Fluorite is the defining mineral for the Mohs hardness value of 4, meaning it is relatively soft compared to many other minerals. This hardness is greater than a copper penny (around 3.5), but less than a standard steel knife blade or window glass (around 5.5).
Attempt to mark the specimen with a common steel object, such as a nail or the tip of a pocketknife. A true fluorite specimen will be easily scratched by the steel, leaving a visible groove, while the steel will not scratch a harder mineral like quartz.
The second physical test is observing the mineral’s cleavage, which describes how a crystal breaks along planes of weakness. Fluorite possesses perfect octahedral cleavage, meaning it breaks cleanly and smoothly along four distinct planes. If fractured, the resulting fragments will be smooth, flat surfaces that meet at predictable angles, often resembling smaller octahedrons. This precise, geometric breakage is fundamentally different from the conchoidal fracture of glass, which results in curved, shell-like surfaces.
Utilizing Fluorescence for Identification
Fluorite is the mineral from which the term “fluorescence” was derived because of its propensity to glow under ultraviolet (UV) light. This optical phenomenon occurs when trace elements within the crystal lattice absorb UV energy and re-emit it as visible light. Fluorescence is a confirming test, not a definitive one, as not all fluorite specimens will glow.
A long-wave UV light source, often called a black light, is typically used, as most fluorite reacts best to this wavelength. The most common fluorescent color observed is a bright blue or violet, but specimens can also glow green, yellow, or white. Observing a blue glow under a black light is a strong positive indicator, but the absence of fluorescence does not automatically mean the specimen is fake.
Comparing Fluorite to Common Imitations
Glass and plastic are frequent counterfeits that lack the perfect octahedral cleavage of fluorite, instead showing the curved breakage pattern of a conchoidal fracture. Glass is usually harder than fluorite and can scratch it, while plastic is significantly softer and often lacks natural fluorescence.
Harder minerals like quartz and softer ones like calcite are also commonly confused with fluorite. Quartz (Mohs 7) will easily scratch fluorite and lacks any distinct cleavage planes. Calcite (Mohs 3) is slightly softer than fluorite, and while it exhibits perfect cleavage, it breaks into rhombohedrons, a tilted, three-sided shape, instead of octahedrons.