What is a double terminated crystal
A crystal with natural faces at both ends, because neither end was attached as it grew. A growth habit, not a twin; 1 of 176 Handbook sheets names it.
A double terminated crystal has natural faces at both ends of its long axis, where most crystals have one free point and one broken or attached base. The phrase is a growth condition: neither end was fixed to a surface while it grew. It is not a twin. Of 176 Handbook sheets in this site's reference set, one uses the phrase: hemimorphite's, hardness 4.5 to 5.
In short
- What is a double pointed crystal called? Doubly terminated, or double terminated. Double domed is an informal variant of the same idea. Collectors call a crystal with no attachment point at all a floater. None of these words describes a twin.
- A termination is where the crystal was free to grow. Most crystals grow out from a surface, so one end is attached and the other develops faces. Faces at both ends mean the crystal grew suspended in something soft, grew out sideways from a point on its prism, or grew freely and later came loose.
- The phrase doubly-terminated appears on 1 of 176 Handbook sheets in this site's reference set: hemimorphite, where the sheet reads doubly-terminated crystals show hemimorphism. The one sheet that mentions double termination does so to say the two ends are not the same.
- In a polar crystal the two ends differ by symmetry. Three of the 176 sheets describe hemimorphic crystals: hemimorphite, point group mm2; schorl and pyrargyrite, both 3m. A double-terminated schorl has different faces at each end, and that difference is a property, not damage.
- Twin flame and tantric twin are crystal-healing trade names, not mineralogy. A twin is a symmetrical intergrowth defined by a twin law, covered on our twinning page. A cut double point is quartz ground to shape, and loses the natural striations the quartz sheet records on prism faces.
| What it is | What made it | What to look for | What to call it |
|---|---|---|---|
| Floater | Grew with no attachment point, suspended in soft material or free in a cavity | Natural faces at both ends; no contact scar anywhere | Doubly terminated crystal, floater |
| Side-attached crystal | Grew outward from a point on its prism face, both ends free | Natural faces at both ends; one scar or broken patch on the side | Doubly terminated crystal |
| Hemimorphic crystal | A polar structure: point group mm2 or 3m in the 3 sheets that say so | The two ends carry different faces | Doubly terminated, hemimorphic |
| Contact or penetration twin | Two individuals related by a twin law | Re-entrant angles; a composition plane | Twin, with the law if known |
| Parallel or random intergrowth | Two crystals touching by chance | No fixed angle between them | Group or cluster |
| Cut or polished double point | A single-ended crystal or rough ground to two points | Polish lines; no prism striations; equal facets meeting in a symmetrical point | A worked object, not a doubly terminated crystal |
What is a double terminated crystal? The definition, and the one thing it tells you
A double terminated crystal is a single crystal with natural terminating faces at both ends. The long axis is the axis in question: for quartz, the c axis, along which the Handbook of Mineralogy records prismatic crystals with the prism {1010} terminated by the two rhombohedra {1011} and {0111}.
The reason the feature matters is growth. A crystal adds material to its free surfaces. Where it rests against a rock wall or another crystal, it cannot grow faces, and the contact becomes a base, usually broken when the specimen is collected. A double termination is therefore a record that no end was fixed during growth. Three ways that can happen: the crystal nucleated suspended in a soft medium such as clay, sediment or a gel-like fill, and grew in every direction; it grew outward from a small contact on its side, leaving both ends free; or it grew attached and later came loose and kept growing. The first is what collectors mean by a floater.
This is why a double-terminated crystal says something about the pocket. It usually means a pocket filled with something soft, or a crystal that was once loose. Look for the scar: a floater has none, a side-attached crystal has one on the prism face. How specimens form generally is on how mineral specimens form, and the practical side of clay-filled pockets is on how to remove clay from a mineral specimen.
Is a double terminated crystal a twin? No, and the word is taken
A double terminated crystal is one crystal, and a twin is two or more crystals grown together in a fixed symmetrical relationship. Searching double terminated crystal brings back crystal-healing vocabulary such as twin flame crystal and tantric twin crystal. These are trade names from a different world. Their twin has nothing to do with a twin law, and neither phrase appears on any of the 176 sheets in this site's reference set.
A mineralogical twin needs a symmetry operation the crystal's own structure does not already contain, applied along a named plane or axis, such as Dauphiné, Brazil and Japan in quartz. The full treatment, and the survey of which species twin how often, is on mineral twinning explained. We will not repeat it here. Growth condition and twinning are independent: a twin can be doubly terminated, and a doubly terminated crystal is usually not a twin.
When the two ends are different: the one sheet that names it
We counted, because the phrase sounds like a textbook term. Across the 176 Handbook sheets in this site's reference set, converted with pdftotext -layout and searched by regular expression for doubly terminated and double terminated in every spelling, one sheet uses it: hemimorphite's. It reads doubly-terminated crystals show hemimorphism. A wider search for hemimorph, excluding hemimorphite's name in other sheets' association lists, adds two more: schorl, commonly hemimorphic, and pyrargyrite, showing hemimorphism.
All three are polar. Hemimorphite's point group is mm2, schorl's and pyrargyrite's 3m, and in those classes the two ends of the unique axis are not related by symmetry. So a doubly terminated crystal of a polar species is expected to end differently at the top and bottom. Hemimorphite's sheet records it as strongly pyroelectric and schorl's as pyroelectric and piezoelectric, which comes from the same polarity. See the hemimorphite and pyrargyrite pages.
Herkimer, stated only as far as we can source it. The quartz sheet lists Middleville, Herkimer County, New York among its localities and says nothing about habit there. The geology.com account cited below describes the crystals sold as Herkimer diamonds as doubly terminated quartz from cavities in the Cambrian Little Falls Dolostone, cavities often lined with drusy quartz and coated with a tarry hydrocarbon. It also notes that similar crystals come from elsewhere and are not rightly called Herkimers. We claim no more than that.
Natural double point or cut and polished: how to tell
A crystal ground to two points is a worked object, whatever it started as. Quartz is the usual case. The quartz sheet records the prism faces as striated perpendicular to [0001], so a natural prism shows fine horizontal lines across the faces, and the two rhombohedra at a natural termination are commonly unequal in size.
Signs of cutting: prism faces that are smooth, with no striations; polish lines under a lens; facets all the same size, meeting in a neat symmetrical point; and identical pieces in a batch. Signs of a natural double termination: striations, unequal faces, growth hillocks, a contact scar, and the ordinary untidiness of something that grew. The general checklist is on how to tell if a crystal is fake, and glued or re-terminated pieces on how to tell if a specimen is repaired.
Neither is dishonest when it is described accurately; a polished point sold as a polished point is fine. The problem is the label. The rest of this section is indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route, and a sharp natural floater with its locality is on it.