• Chemistry: Ca2ZnSi2O7 , Calcium Zinc Silicate.
  • Class: Silicates
  • Subclass: Sorosilicates
  • Group: Melilite
  • Uses: A very minor ore of zinc and as mineral specimens.
  • Specimens

Hardystonite is well known to collectors of fluorescent minerals from Franklin and Sterling Hill, New Jersey. It will fluoresce a dark purple under short-wave ultraviolet light. This fluorescent color is in striking contrast to the normal color of hardystonite. Fluorescence occurs when the ultraviolet light (invisible to humans) imparts energy to some of the atoms in the mineral. This energy is converted by the atom into visible light that we can then see. Hardystonite is usually a dull, unattractive white, but under short-wave UV light it really impresses! The following table shows the more common fluorescing minerals from Franklin and Sterling Hill, New Jersey.

Mineral: Fluoresces: Mineral: Fluoresces: Mineral: Fluoresces:
Barite white Esperite yellow Margarosanite pale violet
Calcite red Fluorite Violet Willemite green
Clinohedrite orange-yellow Hardystonite Violet-blue Wollastonite red

These minerals are sometimes found in association with each other and can make for wonderfully colorful fluorescent specimens. The violet-blue color of hardystonite really stands out among these minerals. The combination of hardystonite, clinohedrite and willemite is considered a mineralogical classic. Hardystonite is a product of the metamorphism of zinc minerals, probably hemimorphite and/or smithsonite, that were caught up in the regional metamorphism that created the Franklin site. Hardystonite is found no where else.


  • Color is white to less commonly pale pink or brown.
  • Luster is vitreous to dull.
  • Transparency: Specimens are translucent to opaque, rarely transparent.
  • Crystal System is tetragonal.
  • Crystal Habits include massive and granular specimens.
  • Cleavage is perfect in two directions and fair in two other directions (all prismatic).
  • Hardness is 3 - 4.
  • Specific Gravity is approximately 3.4+ (above average)
  • Streak is white.
  • Other Characteristics: Fluoresces a dark violet to blue color under short-wave UV light and no fluorescence under long-wave UV light.
  • Associated Minerals include willemite, esperite, zincite, franklinite, calcite and especially clinohedrite.
  • Notable Occurrence is limited to its type locality: North Hill Mine, Franklin, Sussex County, New Jersey, USA.
  • Best Field Indicators are fluorescence, associations, locality and cleavage.
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HARDYSTONITE specimen hrd-1
$ 115.00
Dims: 5.4 x 3.7 x 3.3" (13.5 x 9.4 x 8.3 cm)
Wt: 3 lbs., 7.3 oz. (1.568 kg)
Franklin, New Jersey, U.S.A.
Upon first glance, this cabinet piece looks like a large chunk of granite. However, when viewed under a shortwave UV light, one can see that it is made up of several unusual minerals, such as Hardystonite, willemite, clinohedrite, and franklinite. These minerals are easy to define by their fluorescent colors: Hardystonite makes up most of the material, and shows a dim but pervasive deep violet glow. The willemite occurs in small spots and in thin, straight veins that glow with the standard bright green color, and the clinohedrite is concentrated on only one face, showing a pale orange. The franklinite is distinct through its lack of fluorescence.
no photo
hrd-1 ($115.00)
Franklin, New Jersey, U.S.A.
HARDYSTONITE specimen hrd-2
$ 52.00
Dims: 4.5 x 3.1 x 1.8" (11.4 x 7.8 x 4.5 cm)
Wt: 1 lb., 1.3 oz. (490 g)
Franklin, New Jersey, U.S.A.
This dull brown hand specimen is made up mostly of Hardystonite. The Hardystonite is massive and has a dull, gray-brown color under white light and a dull luster, but glows with a dim, deep violet color under shortwave UV light. A small amount of willemite and calcite are also present, showing their classic bright-green and orange colors, respectively. There is also a large amount of a darker brown material present- it looks like willemite to me, but does not fluoresce.
no photo
hrd-2 ($ 52.00)
Franklin, New Jersey, U.S.A.
HARDYSTONITE specimen hrd-3
$ 30.00
Dims: 3.0 x 2.9 x 2.2" (7.5 x 7.3 x 5.5 cm)
Wt: 15.1 oz. (429 g)
Franklin, New Jersey, U.S.A.
Under normal light, it is not easy to discern the Hardystonite from the calcite and very difficult to discern it from the willemite on this cabinet specimen. However, when bathed in shortwave ultraviolet light, one can easily discern the dull violet fluorescent color of the Hardystonite as compared to the respective bright red and green colors of the calcite and willemite. This material is massive, showing no crystal form, and has a dull white color under white light and a dull, waxy luster at best. Accompanying it and the two other fluorescent minerals are scores of black franklinite crystals, many of which are broken. The franklinites are distinctive in that they do not fluoresce.
no photo
hrd-3 ($ 30.00)
Franklin, New Jersey, U.S.A.
HARDYSTONITE specimen hrd-4
$ 25.00
Dims: 3.8 x 2.6 x 1.9" (9.0 x 6.5 x 4.7 cm)
Wt: 11.9 oz. (336 g)
Franklin, New Jersey, U.S.A.
This cabinet specimen consists mostly of Hardystonite, willemite, franklinite and clinohedrite. The Hardystonite is the most prevalent, and has a pinkish color that is easy to see in normal light. It is massive, showing no crystal form at all, and has a dull, waxy luster. Under shortwave ultraviolet light, its dull violet color is subtle but easy to see. The other minerals are more difficult to isolate, and the ultraviolet light is needed to clearly isolate them- the willemite will show a bright green coloration and the clinohedrite will show a pale orange color. The franklinite will not fluoresce at all.
no photo
hrd-4 ($ 25.00)
Franklin, New Jersey, U.S.A.
HARDYSTONITE specimen hrd-5
$ 150.00
Dims: 4.7 x 3.3 x 2.9" (11.9 x 8.4 x 7.4 cm)
Wt: 3 lbs., 1.7 oz. (1.410 kg)
Franklin, New Jersey, U.S.A.
This large display piece consists almost entirely of massive Hardystonite. The material shows no definable tetragonal form, and has a dull gray color and a dull waxy luster. It is opaque, and appears thoroughly unremarkable under normal light. However, when bathed in shortwave ultraviolet light, it glows a faint violet color. Bright green veins of willemite and a few small orange patches of clinohedrite will also be visible under UV irradiation.
no photo
hrd-5 ($150.00)
Franklin, New Jersey, U.S.A.
HARDYSTONITE specimen hrd-6
$ 90.00
Dims: 4.0 x 3.2 x 3.1" (10.2 x 8.1 x 7.8 cm)
Wt: 1 lb., 10.0 oz. (737 g)
Franklin, New Jersey, U.S.A.
The massive Hardystonite in this cabinet specimen looks like a dull, pale-brown rock mixed in with a darker brown, mottled rock. Its classic dull violet fluorescence is easily visible, however, under shortwave ultraviolet light. Accompanying willemite also shows fluorescence, in the form of a bright green glow. There is also one tiny spot of clinohedrite, which fluoresces orange-yellow. Materials that do not glow are likely either made up of franklinite, with its black color and submetallic luster, or perhaps zincite, which ranges from yellow to brown.
no photo
hrd-6 ($ 90.00)
Franklin, New Jersey, U.S.A.


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