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Double Terminated Crystals: How a Crystal Grows a Point at Each End

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a crystal grows a point at both ends only when nothing blocks either end while it grows. That happens in three settings: inside soft host material such as clay, mud or gypsum (Pecos diamonds, hourglass selenite), loose or barely anchored in an open cavity (Herkimer quartz, which grew near 200 °C), and suspended in molten rock (volcanic quartz). A crystal grown on a rock wall gets one point; the second tip on most shop "double points" was cut.

Which end of a crystal is the problem?

The base. A typical quartz point starts as a seed stuck to the wall of a vein or pocket and builds outward into open fluid. Atoms can only reach the free end, so faces form there; the attached end stays rough, broken or fused to its neighbours. Collectors call that the attachment end, and the vast majority of natural points have one.

A doubly terminated crystal is the exception. Minerals.net defines it plainly as a crystal with a complete termination on both ends. Its close relative is the floater: Minerals.net's floater entry describes a crystal that formed without being attached to a host rock or matrix, and thus fully developed without any contact points. Every true floater of a pointed mineral will be double terminated, but not every double point is a floater; some Herkimer crystals grew double while still touching the cavity wall at a small spot on one prism face.

For quartz, which is trigonal SiO2, the termination is built from rhombohedral faces meeting at a tip, and a second identical set closes the other end. Minerals.net notes in its quartz entry that quartz crystals are usually striated horizontally, and are sometimes doubly terminated. "Sometimes" is the honest word. Most quartz worldwide is wall-grown.

What conditions let a crystal grow both ends?

Three geological settings explain almost every natural double point on the market. They share one requirement: the growing crystal must be surrounded on all sides by something that supplies silica (or whatever the mineral is made of) and gets out of the way as the crystal expands.

1. Growth inside a soft or soluble host

Here the crystal grows from within a sediment or a softer mineral, pushing it aside or replacing it. The Pecos Valley of southeastern New Mexico is the textbook example. A USGS bulletin on the Roswell Resource Area records that doubly terminated quartz crystals, called 'Pecos diamonds' and collected as mineral specimens, occur in Permian rocks along the Pecos River.

According to Rock & Gem Magazine, they come almost exclusively from the Seven Rivers Formation, about 260 million years old, laid down when the area was a belt of coastal salt flats (sabkhas). The quartz is authigenic: it formed in place, and the article reports that the crystals originally formed inside gypsum crystals. When the softer gypsum weathered away, the quartz was left loose on the desert surface. Geologists James L. Albright and Virgil W. Lueth tied their growth to organic material, dolomite formation and the oxidation of sulfides in small pockets. Pecos crystals are known for an unusually high share of stubby, nearly equant "pseudocubic" shapes and a spread of reds, oranges and yellows from included material.

The same principle works for gypsum growing in mud. At Salt Plains National Wildlife Refuge in north-central Oklahoma, the U.S. Fish and Wildlife Service describes about 12,000 acres of salt flats that are the only place in the world where you can dig for hourglass selenite crystals. Those crystals are dug out of loose salt-flat ground rather than broken off a rock wall, the same unattached situation that lets a crystal finish both ends, because soft sediment yields in every direction. How drying brines build gypsum is covered in the evaporites guide.

2. Growth in an open pocket with almost no attachment

The Herkimer diamond district of New York shows the second route. Geology.com states that "Herkimer diamonds" are, by definition, doubly terminated, and that the vugs that hold them can be smaller than a pea or several feet across. A single pocket can contain a few to a few thousand crystals that range in size from a couple of millimeters to over twenty centimeters, with perfect singles, doubles and clusters side by side.

How hot was the water? A summary of the local research at HerkimerHistory.com cites fluid inclusion work on Herkimer diamonds that places the temperature in the range of 200 degree Celsius, and dolomite mineralization in the region at 140 to 170 degrees Celsius. That is warm brine moving through buried dolostone, not magma. Crystals that nucleated on clay, on tarry hydrocarbon or on a speck of dolomite in the cavity could grow out in every direction, and the pocket's fluid kept feeding both ends.

Those fluids leave fingerprints. Geology.com lists solid hydrocarbon particles as the most common inclusion, salt water and liquid petroleum as the most common liquid inclusions, and carbon dioxide as the most common gaseous inclusion. The inclusions guide explains how to read those under a loupe.

3. Growth suspended in a melt

The third route is molten rock. Crystals that start growing in a magma chamber float in liquid, so nothing blocks either end. The petrography atlas by Alex Strekeisen notes that in volcanic rocks quartz phenocrysts are often euhedral as stubby, doubly terminated, often showing rounding or embayment of resorption: the melt partly dissolved them again before eruption.

These volcanic crystals have a twist. Above 573 °C silica crystallizes as high (beta) quartz, which is hexagonal. The open Perkins mineralogy text explains that heating a quartz crystal to just over 573 C turns it into high quartz, and that the change reverses on cooling, so no room temperature beta quartz survives. The crystal keeps its outline. Minerals.net describes the result: bipyramidal hexagons with no prism faces, which collectors call "Beta Quartz" but which are really alpha quartz paramorphs of beta quartz. A tiny bipyramid with no prism between its two pyramids is a melt-grown double point.

Where do natural double terminated crystals come from?

Example Host Growth setting What to expect in hand
Herkimer quartz, New York Cambrian dolostone vugs Warm brine, about 200 °C Very clear, short, bright faces, black hydrocarbon specks
Pecos diamonds, New Mexico Permian gypsum, Seven Rivers Formation Grew inside gypsum in sabkha sediments Small, stubby, often red to orange, many nearly cube-shaped
Hourglass selenite, Oklahoma Salt-flat ground Gypsum dug from loose sediment, not rock Selenite crystals, the refuge's dig-site specialty
"Beta quartz" bipyramids Rhyolite and other silica-rich volcanic rock Crystallized in melt above 573 °C Small hexagonal bipyramids, no prism, often rounded or embayed
Herkimer-type quartz elsewhere Various Similar pocket growth Geology.com lists Arizona, Afghanistan, Norway, Ukraine, and China

That last row matters for buyers. Geology.com is explicit that the look-alikes from other countries cannot rightfully be called "Herkimers"; only material from the Herkimer County area earns the name. Minerals.net places the classic ground in the Mohawk Valley, an area covered by parts of Herkimer, Montgomery, and Fulton Counties.

What do people get wrong about double points?

  • "Double terminated means rare." Natural double points are uncommon compared with wall-grown points, but whole districts produce them by the thousand. Rarity sits in size, clarity and perfection, not in the second tip itself.
  • "A double point is two crystals joined." A true double termination is one continuous crystal. Two points meeting base to base along a seam is either a twin or a repair, and a loupe on the junction tells you which.
  • "Every polished double point is natural." The second tip is easy to grind. Many double points sold as wands, especially in jasper, agate or amethyst, are cut from rough and have polished faces of equal size.
  • "Floaters grew floating in water." Pecos quartz grew inside gypsum and hourglass selenite inside mud; "floater" describes the absence of an attachment, not a crystal drifting in a pocket of liquid.
  • "Herkimer means any clear double." Material from other localities may be superb, but it should be sold under its own locality.

How can you tell a grown double point from a cut one?

Work through the two ends one at a time under a 10x loupe and a single bright light.

  1. Compare the two terminations. Natural tips are rarely identical. Face sizes differ, edges may be slightly stepped, and small secondary faces appear. Cut tips tend to be symmetrical, with every face the same size.
  2. Look at face texture. Grown quartz faces carry growth marks, and the prism faces show the horizontal striations Minerals.net describes. A polished face is mirror-flat, may show fine parallel polishing lines, and the edges are often slightly rounded.
  3. Check where the faces meet the prism. On a natural crystal the termination faces meet the prism faces at crisp, consistent angles. A ground tip can sit at an odd angle to the body.
  4. Read the inclusions. Herkimer-type quartz often carries black hydrocarbon specks or a moving fluid bubble; a cut wand from massive material shows patterns running straight through both tips, which no growth process produces.
  5. Look for a contact scar. Many natural doubles still show a small rough patch on one side where the crystal rested on the pocket wall. That is a point in favour of authenticity, not a flaw to fear.

Choosing a double terminated crystal

Decide first whether you want a grown double or a cut shape, because they are priced and judged differently. For a grown crystal, pay for clarity, sharp undamaged tips at both ends and a stated locality; matrix pieces, where the crystals still sit in the rock that grew them, show the growth setting best. For cut pieces, judge the material and the polish and expect symmetry.

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Frequently asked questions

What is a double terminated crystal?

A single crystal with complete natural points at both ends instead of one point and a broken or attached base. In quartz it usually means growth without a fixed attachment, in soft sediment, inside another mineral, in an open pocket or in a melt. Many shop double points are cut, not grown.

Are Herkimer diamonds always double terminated?

By definition, yes. The trade name covers doubly terminated quartz from the Herkimer County area of New York. The same cavities also produce singles and clusters, and similar double points from other countries should be sold under their own locality names.

Is a floater the same as a double terminated crystal?

Not quite. A floater is any crystal that grew without a contact point on rock or matrix. For pointed minerals such as quartz that usually produces two terminations, but a crystal can be doubly terminated and still touch the wall at a small spot.

How hot was the water that grew Herkimer quartz?

Fluid inclusion studies summarized for the district point to about 200 °C, with associated dolomite mineralization at roughly 140 to 170 °C. That is warm, deeply buried brine rather than magma.

Why are some double points shaped like two pyramids with no sides?

Those are quartz crystals that grew in molten volcanic rock above 573 °C as high quartz, which forms hexagonal bipyramids. On cooling they changed to ordinary quartz but kept the outline, so collectors call them beta quartz.

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