Iroishi Checker
No. 035 / 141
ReferenceREFERENCE
PrehniteContribute a photo

Prehnite

プレナイト · ぷれないと
NaturalGreen
Gemological dataPROPERTIES
Hardness6-6.5
Specific gravity2.90
Refractive index1.616-1.649
Crystal system斜方晶系（球状集合体）
Color rangeCOLOR RANGE

Pale yellow-green through bright apple green, with rare colorless, blue-green, and pink material. The most prized color is a saturated apple green with high transparency; pale yellow-green material is the commercial standard.

UV responseFLUORESCENCE
Long-wave
365 nm
Generally inert; some material shows weak yellow-green
Short-wave
254 nm
Generally inert
Typical inclusionsINCLUSIONS
  • Fibrous radial structure visible in botryoidal material
  • nclusions in transparent crystals
  • nclusions of epidote, datolite, or pectolite (related cavity minerals)
  • aceted stones
Optical characterOPTICAL TRAITS
  • Doubly refractive, biaxial positive
  • Refractive index 1.611–1.669
  • 0.020–0.035
  • Specific gravity 2.80–2.95
  • nclusions
What to look forID POINTS
  1. 01Refractive index 1.611–1.669 — separates prehnite from chrysoprase (1.53–1.54) and aventurine (1.54–1.55)
  2. 02Specific gravity 2.80–2.95 — heavier than chalcedony group (2.58–2.64) and aventurine (2.65–2.85), lighter than jadeite (3.30–3.38)
  3. 03Fibrous radial structure visible at 10× in botryoidal material
  4. 04Botryoidal 'grape jelly' surface morphology in natural rough is diagnostic
Stones it gets mistaken forSIMILAR STONES
Chrysoprase
Chrysoprase
クリソプレーズ
Chrysoprase is chalcedony (microcrystalline quartz) with RI 1.53–1.54, SG 2.58–2.64, and a uniform translucent green from disseminated nickel silicate. Prehnite has RI 1.611–1.669, SG 2.80–2.95, and characteristic fibrous radial structure visible at 10×; it is also doubly refractive while chrysoprase is essentially singly refractive in aggregate. The refractometer and the magnifier give immediate separations.
Care & handlingCARE
  • Mohs 6–6.5 — durable for normal jewelry use, but softer than quartz; avoid stone-on-stone storage and impact
  • Generally safe to warm soapy water; ultrasonic cleaning is usually safe but avoid for heavily included stones
  • Stable to most jeweler's chemicals; avoid hydrofluoric acid (a silicate solvent)
Market notesMARKET
PRICE RANGE

¥1,000–5,000/ct for commercial pale-green or translucent material, up to ¥15,000–30,000/ct for fine bright transparent Malian stones above 5 ct.

Note: No standard treatments are applied — prehnite is sold in its natural state. The chief market discriminator is transparency and color saturation: bright transparent apple-green Malian material above 5 ct commands prices several times above pale, translucent botryoidal material. Cat's-eye prehnite, produced by oriented fibrous inclusions in some Australian and Chinese material, is a small but distinct niche.

BackgroundBACKGROUND

Prehnite is a calcium aluminum silicate (Ca₂Al₂Si₃O₁₀(OH)₂, orthorhombic) of the phyllosilicate (sheet silicate) family — the same broad family as the micas, though with different sheet stacking. It occurs as botryoidal (grape-cluster) masses, fibrous globular aggregates, and rare transparent tabular crystals, almost always in cavities and vesicles of basaltic and metamorphic host rocks. Color ranges from colorless through pale yellow-green to bright apple green and rare blue-green. Mohs 6–6.5, SG 2.80–2.95, RI 1.611–1.669. The 'grape jelly' nickname comes from its characteristic botryoidal habit.

Origin & historyORIGIN & HISTORY

Origins

aceted range is now the benchmark.

History

Prehnite holds a unique place in mineralogical history as the first mineral ever named after a person. The Dutch colonial military officer Colonel Hendrik von Prehn (1733–1785), commander of the Dutch East India Company garrison at the Cape of Good Hope from 1768 to 1780, collected specimens of a then-unknown pale-green mineral on Table Mountain in the early 1770s and sent samples to European mineralogists. The German mineralogist Abraham Gottlob Werner — the dominant geological theorist of the late 18th century, founder of the Freiberg Mining Academy — formally described and named the species in 1788, three years after von Prehn's death, in his honor. The naming established the precedent for the hundreds of mineral species since named for their discoverers, patrons, or honorees. For most of the 19th and 20th centuries prehnite remained a mineralogical curiosity; the 1990s opening of the Sandare deposit in Mali transformed it into a viable gem material.

Lore & symbolism

Lacking deep medieval or classical tradition — the species was identified only in the late 18th century — prehnite's symbolic associations are entirely modern. The 1990s New Age literature describes it as 'the stone of unconditional love,' 'the stone of prophecy,' and 'the healer's stone,' and it is widely sold in Western and Japanese power-stone retail under these positionings. The 'grape jelly' nickname is purely descriptive and dates from the 20th-century American mineral-collector vernacular.

OBSERVATION TOOLS · 3 ITEMS

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References
最終確認日
2026年4月28日
参 考 文 献

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