What is a Herkimer Diamond?

A Herkimer diamond is a unique type of double-terminated quartz crystal discovered within exposed outcrops of dolomite rock, primarily in and around Herkimer County, New York. Despite the 'diamond' in their name, they are not actual diamonds. The term 'double-terminated' refers to their distinctive natural formation with points at both ends. Renowned for exceptional clarity and natural faceting – often displaying 18 facets – these quartz crystals possess a brilliance that mimics true diamonds.

Cluster of Herkimer Quartz diamonds, featuring multiple sparkling, double-terminated quartz crystals naturally formed together. The right photo by: Jeremy Sinkus.

Formation of Herkimer Diamonds

Herkimer Diamonds formed through distinct geological stages over millions of years:

Ancient Host Rock (~500 Million Years Ago): During the Cambrian Period, a shallow sea covered present-day Herkimer County, New York. Sediments from the eroding Adirondack Mountains compacted into the Little Falls Dolostone bedrock.

Cavity Creation: Later, acidic groundwater dissolved parts of the buried Dolostone, forming cavities (vugs) critical for crystal growth.

Crystal Growth: Silica-rich hydrothermal fluids entered these vugs, possibly during the Carboniferous Period (exact timing is debated), triggering quartz crystallization.

Unique Conditions: Suspended in fluid with minimal wall contact, the quartz grew slowly, developing points at both ends—unlike typical quartz attached at one end. This double-terminated shape, along with exceptional clarity, resulted from stable, silica-rich conditions and organic inclusions.

The exceptionally slow, undisturbed crystallization within this specific geochemical environment resulted in the high clarity and vitreous luster for which Herkimer Diamonds are known. Thus, while the host dolostone is Cambrian, the crystals themselves formed significantly later within these pre-existing vugs due to subsequent geological processes.

Though the Dolostone is ancient, the Herkimer Diamonds are younger, formed by dissolution and slow crystallization in protected cavities.

Herkimer diamonds embedded in dolomite matrix, from Ace of Diamonds Mine, Middleville, Town of Newport, Herkimer County, New York.

Key Characteristics of Herkimer Diamonds

Herkimer Diamonds possess several unique features that distinguish them from other quartz crystals and contribute to their value and appeal.

Composition: Fundamentally, a Herkimer Diamond is a type of quartz, composed of silicon dioxide (SiO₂), the same mineral as amethyst and clear rock crystal.

Double Termination: Herkimer quartz has two naturally formed, pointed ends, or terminations, rather than the more common single termination where one end tapers to a point and the other attaches to a base or matrix.

Crystal Structure & Natural Faceting: Herkimer Diamonds belong to the hexagonal crystal system. They naturally form with a specific geometry, typically featuring 18 facets: six facets along the central body (prism faces) and six facets tapering to a point at each of the two terminations.

High-quality Herkimer diamonds, showcasing their exceptional clarity and unique double-terminated quartz crystal structure, from Treasure Mountain Diamond Mine, Little Falls, Herkimer County, New York, USA. Photo by: Adam Scanlon.

Appearance (Clarity, Luster, & Color)

Clarity: They are renowned for their exceptional clarity, often appearing "water-clear" or nearly flawless to the naked eye.

Luster: Herkimers exhibit a bright, glassy (vitreous) luster.

Color: While often colorless, their color can range to smoky gray or even black. This coloration is frequently influenced by inclusions, particularly hydrocarbons.

Brilliance: While clear and sparkly due to their facets and luster, they lack the intense fiery brilliance (dispersion) seen in true, cut diamonds.

Hardness: Like all quartz varieties, Herkimer Diamonds rate a 7 to 7.5 on the Mohs hardness scale.

Forms and Shapes Variations

While many Herkimer Diamonds are relatively small—often less than an inch in length—their sizes can vary significantly. Though they are best known for their classic double-terminated form, these quartz crystals can also exhibit a range of fascinating morphological variations, including:

• Skeletal Crystals: Featuring internal cavities or hopper-like growth patterns, often due to rapid or uneven crystallization.
• Twinned Crystals: Two or more crystals intergrown in a specific crystallographic orientation, forming symmetrical, often mirror-like structures.
• Clusters: Groups of individual double-terminated crystals that have naturally grown together.
• Phantom Crystals: Showing ghost-like outlines of earlier growth phases, visible within the body of the crystal.
• Rare Forms: Occasionally, Herkimer Diamonds exhibit uncommon shapes such as:
• Scepters: Where a larger crystal cap sits atop a thinner, stem-like base.
• Barbells: Crystals that grow outward from a central point, giving them a barbell-like appearance.

Herkimer Diamond Size & Shape Variations. Photos by: Frank J McGivern Jr

Inclusions in Herkimer Diamonds

Herkimer Diamonds often contain fascinating inclusions—materials or voids trapped during crystal growth—that add both beauty and scientific value. Common types include: 

Black Hydrocarbons (Anthraxolite): Solid carbon-rich flecks, often giving a smoky appearance, and are among the most common inclusions.

Liquid Inclusions: Typically saltwater or petroleum, sealed in microscopic cavities, offering clues about past chemical environments.

Gaseous Inclusions: Mostly carbon dioxide bubbles, indicating gaseous conditions during formation.

Two-Phase and Three-Phase Inclusions: Combinations of gas, liquid, and sometimes solids, providing complex formation insights.

Enhydro Inclusions: Rare water bubbles visible inside the quartz, adding uniqueness.

Negative Crystals: Hollow, crystal-shaped voids formed naturally during growth.

Mineral Inclusions: Such as calcite, dolomite, pyrite, sphalerite, and quartz, each contributing to the crystal's appearance and value.

Herkimer Diamonds often contain a range of fascinating inclusions—materials trapped inside the crystal during growth—that enhance both their scientific and aesthetic value.

Herkimer Diamonds displaying various inclusions: black hydrocarbons (anthraxolite), liquid inclusions (saltwater and petroleum), rare enhydro water bubbles, and negative crystal.

Identifying Herkimer Diamonds

Here are some key features to look for when identifying these exceptional crystals:

Double-Terminated Structure: Herkimer Diamonds are characterized by their double-terminals—crystals with pointed ends at both ends, creating a symmetrical and visually striking shape.

Hexagonal Crystal Shape: These crystals typically have a hexagonal cross-section and prismatic shape, with six sides that reflect light and add to their natural beauty.

Natural Faceting: One of the most distinctive features of Herkimer Diamonds is their natural faceting, which gives them a polished appearance without the need for human intervention.

Clarity and Transparency: Herkimer Diamonds are known for their exceptional clarity, often appearing transparent or nearly so with minimal inclusions.

Herkimer Diamonds in various forms: skeletal crystals, twinned crystals, and clusters.

Why Are Herkimer Diamonds Called “Diamonds”

Though they are quartz crystals, Herkimer diamonds are so named because of their exceptional clarity and naturally occurring faceting, which give them a brilliance similar to that of true diamonds. Unlike true diamonds, which are composed of carbon, these crystals are chemically quartz.

They are also occasionally referred to as “Middleville diamonds” or “Little Falls diamonds,” names that derive from the locations where they were first discovered.

Herkimer Diamond Vs. Real Diamond: Key Differences

Composition:

• Herkimer Diamond: A type of double-terminated quartz (silicon dioxide, SiO₂) found in Herkimer County, New York.
• Real Diamond: Composed of carbon (C), formed under high pressure and temperature deep within the Earth.

Hardness:

• Herkimer Diamond: Mohs hardness of ~7 (typical of quartz).
• Real Diamond: Mohs hardness of 10 (hardest natural material).

Refractive Index & Brilliance:

• Herkimer Diamond: Lower refractive index (~1.55), resulting in less brilliance and a glassier appearance.
• Real Diamond: High refractive index (~2.42), producing exceptional sparkle and "fire."

Crystal Structure:

• Herkimer Diamond: Naturally double-terminated (pointed at both ends) with hexagonal prisms.
• Real Diamond: Typically forms in octahedral crystals; cut and polished into facets for jewelry.

Density/Specific Gravity:

• Herkimer Diamond: ~2.65 g/cm³ (lighter for its size).
• Real Diamond: ~3.5 g/cm³ (feels heavier).

Origin & Value:

• Herkimer Diamond: Sourced primarily from Herkimer County; affordable and used in decorative jewelry.
• Real Diamond: Mined globally (e.g., South Africa, Russia); high value due to rarity and demand.

Identification Tips:

• Visual Clues: Herkimers lack the fiery brilliance of diamonds and may have inclusions like water bubbles.
• Professional Tests: Thermal conductivity testers, scratch tests (caution advised), or specific gravity measurements.
• Jeweler’s Tools: Examine under magnification for facets, inclusions, and light performance.

Herkimer Diamond identification guide showing key features: double-terminated quartz points, natural geometric faceting, and high clarity with transparent appearance.