Blue Apatite: The Mohs 5 Mineral and How to Care for It

Short answer: blue apatite is a calcium phosphate, almost always fluorapatite (Ca5(PO4)3F), and it is the mineral that defines 5 on the Mohs hardness scale. A steel knife scratches it. Specific gravity runs about 3.1 to 3.35. Gem blue and blue-green material comes mainly from Brazil, Myanmar, Sri Lanka and Madagascar, heat is often used to improve blue color, and the stone is sensitive to acid, heat and knocks, so it needs gentle care.

Why is apatite the number 5 on the Mohs scale?

The Mohs scale ranks minerals by scratch resistance against ten reference minerals, and apatite holds the slot between fluorite and orthoclase feldspar. Geology.com's apatite page explains that apatite has a relatively consistent hardness and serves as the index mineral for 5 on the scale, and its Mohs scale page lists the full sequence: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10.

That position matters to a buyer in two ways. First, Geology.com notes apatite can be scratched with a steel knife blade, so a knife test that would leave quartz untouched will mark it. Second, ordinary quartz dust (7) is harder than apatite, so a blue apatite tumble rubbing against quartz points in a bag will scuff. The hardness testing guide covers how to run scratch tests without damaging a specimen.

What is apatite, chemically?

"Apatite" is a group name, not a single mineral. Minerals.net describes the primary group as three hexagonal phosphates that differ in one position of the formula:

Minerals.net adds that collectors and dealers rarely specify which member they have and simply say "apatite," and that most fine crystals are fluorapatite. Webmineral's fluorapatite entry works out the composition: 39.74% calcium, 18.43% phosphorus and 3.77% fluorine by weight, with a calculated density of 3.20.

The name is a small joke at mineralogists' expense. The International Gem Society traces it to the Greek apatein, "to deceive," because early mineralogists confused apatite with other species. Webmineral gives the root as apatao, glossed as being misleading.

Why does apatite matter outside the crystal shop?

Because it feeds the world. Minerals.net calls apatite the most common phosphate mineral and the main source of the phosphorus plants need. Geology.com explains that phosphate rock, a sedimentary rock with at least 15 to 20% phosphate by weight, gets that phosphorus from fine-grained apatite, and that most of it becomes fertilizer. The USGS states that phosphate rock minerals are the only significant global resources of phosphorus.

How does blue apatite form?

Apatite forms almost everywhere rocks do, but the colorful crystals come from specific settings. Minerals.net lists gemmy crystals in granite pegmatites, hornfels in contact metamorphic rocks, diabase, ore veins and nepheline syenite pegmatites. Geology.com notes that the largest deposits are sedimentary, built from phosphate-rich organic debris such as bones, teeth and scales, while the well-formed, vivid crystals cut for gems come from hydrothermal veins and pegmatite pockets.

IGS records apatite in pegmatites, hydrothermal veins and cavities, metamorphic rocks, and as detrital grains in sediments. Its habit list includes prismatic and stubby crystals as well as massive, granular and compact forms. The opaque blue tumbles and palm stones sold in crystal shops are cut from massive material rather than clear crystals, which is why they look like a dense blue rock rather than a gem.

Where does blue apatite come from?

IGS names Brazil, Canada, India, Madagascar and Mexico as the producers of most gem-quality apatite, and sorts sources by color:

• Blue: Brazil, Myanmar, Sri Lanka.
• Neon blue-green: Madagascar. IGS credits this find with pulling apatite into jewelry, and says the best stones can approach the color of Paraíba tourmaline.
• Blue-green (moroxite): Arendal, Norway.
• Yellow: Durango, Mexico, the classic Cerro de Mercado crystals.
• Violet: Germany, Portugal, Maine and California.

Minerals.net adds specimen localities for blue: deep blue crystals from Sludyanka near Lake Baikal in Russia, and deep blue crystals from Ipirá in Bahia, Brazil. For the wider picture of Madagascar's output, see the planned Madagascar origin guide.

Size is the limiting factor for blue. IGS notes that blue faceted gems from Brazil are almost always 1 to 2 carats, Myanmar produces blue stones up to about 10 carats, and blue-green stones rarely exceed 5 carats.

How do you recognize blue apatite?

• Hardness: 5. A steel knife scratches it.
• Cleavage: poor to indistinct; Geology.com describes it as brittle and often highly fractured.
• Luster: vitreous to subresinous, per Geology.com.
• Pleochroism: IGS says gem blue apatite shows strong blue and yellow dichroism, a quick tell in transparent pieces.
• Fluorescence: IGS reports that blue apatites fluoresce violet-blue to sky blue under UV.
• Acid: Minerals.net notes apatite dissolves in hydrochloric acid. Do not test a specimen you care about this way.

What gets confused with blue apatite?

Minerals.net's comparison list: beryl, tourmaline and quartz are harder; calcite is softer and fizzes in acid. The confusion that matters most in the shop is with aquamarine and with blue or teal fluorite. Aquamarine is 7.5 to 8 and will scratch apatite; fluorite is 4 and has perfect octahedral cleavage that leaves flat, triangular breaks. A planned guide compares aquamarine and blue apatite side by side. With faceted stones, IGS warns that apatite and tourmaline overlap in many properties and can be hard to separate without a destructive hardness test.

Is blue apatite treated?

Often, for faceted stones. IGS states that heating can improve color, especially for blue and blue-green stones, and Geology.com's photo of Madagascar apatites labels a blue and a bluish green oval as heat treated. Synthetic apatite also exists: IGS notes that stones grown for laser research have reached the gem trade and show a color change effect that natural apatite does not.

How should you care for blue apatite?

IGS is blunt: apatites are acid, heat and shock sensitive and deserve the same care as opal. In practice:

1. Wipe, do not soak. A damp cloth and a quick dry. Avoid acidic cleaners, vinegar and lemon-based products, since apatite dissolves in acid. The water-safe crystals guide covers soaking in general.
2. No ultrasonic or steam cleaning. IGS says never use mechanical cleaning systems on apatite.
3. Keep it away from heat. IGS rates its heat sensitivity as "very." Not on a radiator or a sunny dashboard.
4. Store it separately. Quartz and most other tumbles are harder and will scratch it.
5. Choose protective jewelry settings. IGS recommends earrings and pendants over rings, and protective settings for occasional wear.

What is blue apatite associated with in crystal traditions?

In crystal traditions blue apatite is associated with motivation, focus and communication. These are traditional associations, not properties of the mineral.

What to look for when you buy blue apatite

Most blue apatite sold as tumbles, palm stones and free forms is opaque massive material, so judge it on color and surface rather than clarity.

1. Color: a saturated, even blue to blue-green. Gray patches and white streaks are other minerals in the rock.
2. Polish: at hardness 5, a well-polished stone is a sign of careful finishing; look for drag marks and pits in close-up photos.
3. Chips: check edges and corners, which take knocks first on a brittle stone.
4. Size versus weight: at a specific gravity around 3.1 to 3.2, blue apatite feels noticeably heavier in the hand than quartz (about 2.65) of the same size, which helps rule out dyed quartz or glass.

• Blue Apatite palm stones and tumbles: small palm stones from about 1 to 2 inches plus large tumbles, an affordable way to see the color and feel the weight.
• Blue Apatite free forms: free-form pieces about 2.25 to 2.75 inches long, enough polished surface to judge color evenness and finish quality.
• Aquamarine rough: rough aquamarine chunks and pieces in feldspar, the harder blue mineral most often compared with apatite.
• Fluorite (Rainbow): fluorite at hardness 4, one step below apatite on the Mohs scale, for a side-by-side scratch and cleavage comparison.

Frequently asked questions

Is blue apatite a real mineral or a trade name?

It is a real mineral. Blue apatite is apatite, almost always fluorapatite (Ca5(PO4)3F), in a blue color. "Apatite" itself is a group name covering fluorapatite, chlorapatite and hydroxylapatite, and dealers rarely specify which one they are selling.

Can blue apatite go in water?

A quick rinse or damp cloth will not hurt it, but long soaking is not recommended. Apatite dissolves in acid, so never use vinegar or lemon water, and avoid ultrasonic or steam cleaners. Dry it promptly and store it away from harder stones.

Is blue apatite good for a ring?

Not for daily wear. At hardness 5 it scratches easily, and IGS describes apatite as brittle and sensitive to shock. Choose earrings or a pendant, or a ring with a protective setting worn occasionally.

Where does the best blue apatite come from?

For faceted blue gems, IGS lists Brazil, Myanmar and Sri Lanka, with Madagascar known for neon blue-green. Brazilian blue gems are usually only 1 to 2 carats, while Myanmar has produced blue stones up to about 10 carats.

How do you tell blue apatite from blue fluorite?

Try hardness and breakage. Fluorite is 4 on the Mohs scale and splits along perfect octahedral cleavage, leaving flat triangular faces. Apatite is 5 with poor to indistinct cleavage, so its breaks are uneven. Under UV, blue apatite tends to fluoresce violet-blue to sky blue.

Sources

• Geology.com, Apatite: https://geology.com/minerals/apatite.shtml
• Geology.com, Mohs Hardness Scale: https://geology.com/minerals/mohs-hardness-scale.shtml
• Minerals.net, Apatite: https://www.minerals.net/mineral/apatite.aspx
• International Gem Society, Apatite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/apatite-jewelry-and-gemstone-information/
• Webmineral, Apatite-(CaF) Mineral Data: http://webmineral.com/data/Apatite-%28CaF%29.shtml
• USGS, Phosphate Rock Statistics and Information: https://www.usgs.gov/centers/national-minerals-information-center/phosphate-rock-statistics-and-information