Overview

Ettringite is a hydrated calcium aluminum sulfate, Ca6Al2(SO4)3(OH)12·26H2O, celebrated by collectors for its luminous lemon-yellow color and elegant hexagonal prisms. Crystals range from delicate acicular sprays to stout, glassy prisms with flat pinacoidal terminations. The Kalahari Manganese Field (South Africa) set the world standard, producing saturated yellow columns with brilliant luster on contrasting black manganese-oxide matrix. Type-locality material from the Eifel Volcanic Field (Germany) shows classic silky radiating needles, while historic finds from Crestmore (California) and Monteponi (Sardinia) offer fine, often colorless to pale-yellow sprays.

Despite its beauty, ettringite is a fragile, highly hydrated sulfate. Prisms chip easily, and the species can be sensitive to heat and prolonged water exposure. Intact, lustrous crystals with strong yellow color—especially on aesthetic matrix—are scarce and command the highest interest.

Popularity

Ettringite’s appeal is anchored in its color and form: a glowing, cheerful yellow uncommon among sulfates, paired with crisp hexagonal geometry. It fills an important niche in collections focused on the Kalahari Manganese Field, hydrated sulfates, or type-locality suites. While micro-sprays and small radiating clusters are widely attainable, truly top cabinet pieces—undamaged, saturated yellow prisms, ideally on contrasting matrix—are quite rare and keenly sought. Because crystals are brittle and mining environments can be rough, many notable specimens are repaired; unrepaired matrix pieces with multiple perfect terminations are prized and can be surprisingly expensive relative to the species’ overall abundance.

Top Collecting Localities

Wessels Mine, South Africa

Wessels Mine (Kalahari Manganese Field, Northern Cape) is the benchmark for world-class ettringite. It produced exceptionally sharp, lemon-yellow hexagonal prisms—often 1–5 cm, sometimes larger—with glassy to silky luster and textbook flat terminations. Jet-black manganese-oxide matrix provides striking contrast, and associations can include fellow Kalahari rarities such as sturmanite and charlesite. Top Wessels pieces display radiating clusters or freestanding prisms perched just-so on sculptural matrix. Because of extraction stresses, repairs are common; unrepaired, undamaged matrix specimens command premiums.

N'Chwaning II Mine, South Africa

N'Chwaning II is another Kalahari icon, yielding vivid yellow to canary ettringites—often as elegant, elongated prisms with excellent translucency and silky luster. Clusters may show subtle color zoning and robust, sharp terminations. The best pieces feature airy, three-dimensional arrangements on dark matrix, with crystals spaced to highlight form and color. While overall production was not immense, the consistent quality from this mine makes N’Chwaning II ettringites staples of advanced Kalahari suites.

Collector’s Guide

Evaluating Specimen Quality

• Color and saturation:

  • The most desirable ettringite shows a bright, saturated lemon- to canary-yellow. Pale or colorless is common and still collectible when paired with elegant form or historic provenance, but intense, uniform yellow earns the highest marks.
  • Backlighting can intensify color, but top specimens still “read” yellow under normal display lights.

• Crystal size and form:

  • Kalahari prisms: stout to elongated hexagonal prisms with flat, sharp terminations and bright luster are the gold standard.
  • Micro to miniature sprays (Eifel, Crestmore, Monteponi) should be well-defined and symmetric, ideally in isolated, undamaged rosettes or radiating clusters.

• Luster and clarity:

  • A glassy to silky luster on prism faces and terminations is ideal. Dull, etched, or heavily frosted surfaces are less desirable unless compensated by excellent color or rarity.
  • Slight translucency that glows at the tips is a plus.

• Matrix and composition:

  • Contrasting matrix dramatically boosts aesthetics: black manganese oxides (Kalahari), blue calcite (Crestmore), or clean, white/gray host rock (Eifel, Monteponi).
  • Well-balanced compositions—crystals oriented and spaced attractively—display better and command higher prices.

• Condition:

  • Tips chip easily. Inspect all terminations and edges under a loupe; tiny white nicks are common and reduce value.
  • For sprays, look for intact, complete hemispheres without broken needles, especially on the display side.
  • Many Kalahari pieces are repaired; a single, discrete repair on a major crystal can be acceptable if disclosed and not distracting. Unrepaired pieces are premium.

• Rarity and provenance:

  • Wessels and N’Chwaning II “showpiece” clusters with saturated color are scarce and highly liquid in the market.
  • Type-locality Eifel material and historic Crestmore pieces are important for pedigree-focused collections.

Detecting Repairs or Treatments

• Repairs and reattachments:

  • Very common on Kalahari specimens. Look for:
    • Perfectly straight or planar seams across a crystal.
    • Slight misalignment in striations at a junction.
    • Glossy glue lines at the crystal–matrix interface, often visible under UV (some epoxies fluoresce).
  • Sprays may be stabilized with dilute consolidants; look for a subtle, uneven sheen in recesses.

• Polishing and coatings:

  • Polished terminations are unusual and generally discouraged; a mirror-like, “too perfect” flat might indicate polishing.
  • Avoid specimens with obvious clear coatings used to enhance luster; these can yellow or peel with time.

• Assemblies:

  • Beware of crystals mounted on unrelated matrix. Mismatched matrix minerals or obvious adhesive residues at contact points are red flags.

When in doubt, buy from reputable sources and ask specifically about repairs and any stabilizers used.

Care and Storage

Ettringite is a hydrated sulfate and comparatively fragile. Proper care preserves color, luster, and integrity.

• Handling:

  • Handle with two hands by the matrix, never by the crystals. Ettringite has low hardness (~2–2.5) and can chip or snap easily.
  • Transport in snug, padded containers; avoid vibration and stacking.

• Environment:

  • Keep temperature moderate and stable; avoid heat sources and direct sunlight—heat can dehydrate or craze the crystals.
  • Avoid extremes of humidity. A closed display case with gentle, stable indoor humidity is ideal. Do not store with aggressive desiccants that could overdry the specimen.

• Light:

  • Normal LED case lighting is fine. Prolonged strong sunlight is not recommended due to heat buildup and potential dehydration.

• Cleaning:

  • Prefer dry cleaning: soft air bulb or a very soft brush to remove dust.
  • If absolutely necessary, use a barely dampened swab with distilled water to touch only the matrix—not the crystal faces—and dry immediately. Prolonged contact with water can dull or dissolve surfaces; never soak.
  • Avoid acids, ultrasonic cleaners, steam, and harsh chemicals.

• Storage:

  • Display in a dust-free cabinet, secured with a tiny amount of museum putty on the matrix (not touching crystals).
  • For boxed storage, wrap loosely in acid-free tissue with additional foam around the matrix to prevent pressure on crystals.
  • Periodically inspect for any new micro-chips or changes in luster; address dust accumulation gently and promptly.

With mindful handling and a stable environment, fine ettringites—especially the luminous Kalahari classics and well-preserved type-locality sprays—will retain their beauty for generations.