Mohs Hardness 5 to 5.5: Apatite, Obsidian and Turquoise

Gems at Mohs 5 to 5.5 sit well below the critical Mohs 7 dust threshold and require significant care and protective settings for use in jewelry. Despite their relative softness, several beautiful and historically important gem materials fall in this range, including turquoise, apatite, and obsidian. Understanding their limitations helps you enjoy these gems while protecting them from damage.

Turquoise at Mohs 5 to 6

Turquoise is a hydrated copper aluminum phosphate mineral with a hardness of Mohs 5 to 6, varying with quality and porosity. It is one of the oldest gem materials in human history, prized by ancient Egyptians, Persians, Native Americans, and many other cultures for thousands of years. Its distinctive blue to blue-green color is caused by copper, while green tones indicate more iron content.

Turquoise is porous, meaning it absorbs liquids and oils. This porosity is both a vulnerability and the reason turquoise is often stabilized or treated. Natural untreated turquoise is the most valuable but also the most sensitive. Stabilized turquoise has been impregnated with resin to improve hardness and color stability. Treated turquoise may be dyed or coated.

Care for turquoise is strict: clean only with a dry or barely damp soft cloth. Never use soap, chemical cleaners, ultrasonic cleaners, or steam. Never soak turquoise in water. Keep away from perfumes, oils, and cosmetics, which can permanently stain the porous surface. Store away from direct sunlight, which can fade the color. Turquoise is best suited for earrings, pendants, and brooches rather than rings and bracelets.

Apatite at Mohs 5

Apatite is calcium phosphate, the same mineral that makes up tooth enamel and bone. As a gem, it occurs in vivid blue, green, yellow, and pink colors. Neon blue-green apatite from Madagascar and yellow apatite from Mexico are particularly prized. Apatite has Mohs 5 hardness and poor toughness with imperfect cleavage.

Apatite is too soft for rings and bracelets worn daily. It is best used in earrings and pendants where it experiences less contact and impact. Clean with a soft damp cloth. Avoid ultrasonic cleaners and chemical cleaners. Handle carefully to avoid scratching and impact. Despite its softness, apatite's extraordinary colors make it a favorite among gem collectors.

Obsidian at Mohs 5 to 5.5

Obsidian is volcanic glass, not a true mineral, but it is widely used as a gem and ornamental material. It forms when lava cools rapidly, preventing crystal formation. Obsidian is typically black but also occurs in mahogany (black and brown), rainbow (with iridescent sheen), snowflake (with white cristobalite inclusions), and gold and silver sheen varieties.

Obsidian has Mohs 5 to 5.5 hardness and no cleavage, but it fractures conchoidally (like glass) and can produce extremely sharp edges. It is brittle and chips easily from impact. Obsidian is best suited for pendants, earrings, and decorative objects rather than rings. Clean with a soft damp cloth. Avoid ultrasonic cleaners and impact.

Other Mohs 5 to 5.5 Gems

Lazurite, the blue mineral in lapis lazuli, has a hardness of about Mohs 5 to 6. Lapis lazuli as a rock (which also contains calcite and pyrite) has variable hardness. Sodalite is Mohs 5.5 to 6. Smithsonite is Mohs 4 to 4.5. Hemimorphite is Mohs 4.5 to 5. All of these require gentle care similar to turquoise and apatite.

Frequently Asked Questions

Can turquoise be worn in a ring?

Turquoise rings are traditional in many cultures, particularly in Native American and Southwestern jewelry. However, turquoise at Mohs 5 to 6 will accumulate surface scratches with daily ring wear. If you wear a turquoise ring, choose a protective bezel setting, remove it before any physical activity, and accept that the surface will show wear over time.

Why does turquoise change color?

Natural untreated turquoise can change color over time due to absorption of oils, cosmetics, and other substances into its porous structure. Exposure to light can also cause color changes. Stabilized turquoise is more resistant to color change. Keeping turquoise away from oils, cosmetics, and direct sunlight slows color change.

Is apatite related to tooth enamel?

Yes. Tooth enamel and bone are composed primarily of hydroxyapatite, a form of apatite. The name apatite comes from the Greek word for deceit, because apatite was frequently mistaken for other minerals including beryl, tourmaline, and olivine.

Conclusion

Gems at Mohs 5 to 5.5 including turquoise, apatite, and obsidian require careful handling, protective settings, and gentle cleaning. Their softness limits their practical use in rings and bracelets but does not diminish their beauty or historical significance. With appropriate care and setting choices, these gems can be enjoyed safely in earrings, pendants, and other lower-impact jewelry applications.