Quick answer: A double-terminated crystal has natural points on both ends because it grew without being attached to rock, suspended in soft sediment or loose in a cavity. Herkimer diamonds from New York are the most famous double-terminated quartz. Natural terminations show growth marks and striations; polished points look glassy-perfect.
Look at a typical quartz point and you will see a six-sided prism with a pointed tip at one end and a rough, broken, or flat base at the other. That base is where the crystal was attached to rock while it grew. A double-terminated crystal has a natural point at both ends, and that tells you something unusual about how it formed.
Want the full picture? See Quartz: The Complete Guide.
Why most crystals have only one point
In a typical pocket, crystals start growing on the wall of a cavity. The wall supplies the first attachment point, and the crystal grows outward into open space. Only the free end can develop faces, so the attached end stays rough. When the crystal is removed, the base is either broken or still attached to matrix, a point covered in what “on matrix” means.
How a crystal gets two points
To grow two terminations, a crystal must be free to grow in both directions. That happens in a few ways:
- Growing suspended in soft sediment. In some clays, muds, and evaporite beds, crystals grow while surrounded by soft material rather than attached to a hard wall. They can develop faces all around. The “Pecos diamonds” of New Mexico, double-terminated quartz that grew inside gypsum beds, are a classic example.
- Growing with a tiny point of attachment. If a crystal starts on a single small grain or is held only lightly, both ends can keep growing.
- Growing loose in a cavity. In some pockets crystals seem to have detached early and kept growing while resting on clay or other crystals.
Herkimer diamonds: the famous case
New York’s Herkimer diamonds are the best-known double-terminated quartz. They grow in cavities in the Little Falls Dolostone of the Mohawk Valley, and a high proportion are doubly terminated, water-clear, and naturally brilliant, which is why they were nicknamed “diamonds.” Our posts on the real story behind Herkimer diamonds and the Mohawk Valley rock layer explain why. Black and smoky Herkimers, colored by carbon-rich inclusions, can also be doubly terminated.
Natural versus polished terminations
Many double points on the market are cut and polished from larger quartz rather than grown that way. They are still quartz, just shaped by people, like a polished cabochon. To tell the difference:
- Natural faces often show tiny growth features: fine horizontal striations on the prism faces, small triangles or steps on the tip faces, and slightly uneven surfaces.
- Polished points tend to have perfectly flat, glassy faces, very even angles, and sometimes faint polishing marks or rounded edges.
- Face count and shape on natural quartz follow the crystal’s geometry but are rarely perfectly symmetrical, because growth conditions are never perfectly even.
Our post on crystal habit explains why no two natural crystals look quite the same, and why crystals grow in geometric shapes covers the underlying structure.
Smoky double-terminated quartz
Double terminations also occur in smoky quartz, where natural radiation has darkened the crystal after growth, as described in smoky quartz. A dark crystal with two sharp tips shows both growth freedom and a long post-growth history.
Quick answers about double-terminated quartz
What does “double terminated” mean?
A double-terminated crystal (also called doubly terminated) has a natural point, or termination, at both ends. Most quartz crystals grow attached to rock at one end, so they have one point and one broken base. A double-terminated crystal grew without that attachment, suspended in clay, mud, or soft sediment, or floating loose in a cavity, so it could build a point in both directions.
How can you tell a natural double termination from a polished one?
Natural faces carry growth marks. On quartz, the long prism faces usually have fine horizontal lines (striations) running across them, and the point faces often show small triangles, steps, or slightly uneven luster. A polished or carved point tends to be glassy-perfect, with crisp, identical facets and no striations at all. Polished points are perfectly fine to own, they just are not natural terminations.
Are double-terminated crystals rare?
Less common than single-terminated crystals, yes, but not rare as a category. Water-clear, sharp, gemmy double-terminated crystals are the prized ones, and Herkimer County, New York, is the most famous source in the world. Read the complete Herkimer diamonds guide to see why that locality produces so many.
Can other minerals be double terminated?
Yes. Calcite, aragonite, gypsum (selenite), and many other minerals can form doubly terminated crystals whenever they grow free of a base. Quartz just does it most visibly.
A few double-terminated pieces in the shop: Large Double-Terminated Herkimer Diamond Quartz — Fort Plain, New York, Double Terminated Quartz Crystal Set — 3 Natural Specimens, and Double Terminated Smoky Quartz Point — Natural Crystal.
