Trigonal-trapezohedral:
The forms to name in this class are nacoid, trigonal prisms, hexagonal prism, ditrigonal prisms, and rhombohedrons. Quartz is the common mineral to name in this class.
Rhombohedral: ...
Trigons
Trigons are triangular patterns that crystallized in the Diamond when the Diamond was born millions of years ago. It's like rings in a tree. They show how it's grown.
Trigon - A triangular indentation occurring as a growth mark on diamond octahedron faces. The sides of the trigon are reversed with respect to the face on which it occurs.
trigon Triangular shaped impressions on the skin of the rough or a natural on a finished diamond which also indicate granular orientation for the cutter.
The Trigonal Division
Now we have worked through the first 7 classes in the Hexagonal System, all having some degree of 6-fold symmetry. Time to shed that 6-fold symmetry and look at the Trigonal Division of the Hexagonal System.
trigonal (pyramidal or tabular, usually flattened on c-axis and somewhat triangular in shape)
Envronment:
Two Locals San Benito Co. Calif. and Mont. St. Hilaire, Quebec, Canada ...
trigonal, hexagonal (trigonal-trapezohedral)
Crystal Habit
enantiomorphic, prismatic ...
Trigonal
Sapphire
Modern Birthstone - Pisces, Virgo, Taurus, Sagittarius ...
Trigon, Trigons
Triangular marks, usually small, and quite common on surfaces of rough and polished diamonds, due to twinning and other crystal growth factors.
Twin, Twinned, Twinning ...
TRIGONAL OR (RHOMBOHEDRAL CRYSTAL SYSTEM) - One of three-fold symmetry crystal growth or uniaxial. Many gemstones commonly used in jewelry have this type of crystal system. Examples include as tourmaline, quartz, calcite and corundum.
Hexagonal-trigonal
Colors
Any and all. Tourmaline occurs in more colors than any other gem. Some colors have specific variety names, including: ...
Dolomite group: Trigonal
Ankerite CaFe(CO3)2
Dolomite CaMg(CO3)2
Minrecordite CaZn(CO3)2
Barytocite BaCa(CO3)2 ...
Crystal System: Trigonal
Luster: Vitreous
Density: 2.61 (inclusions generally increase this) ...
Crystallize in the trigonal sect of the hexagonal system, and most commonly form rhombohedrons and scalenohedrons.
2. Have perfect rhombohedral cleavage
3.
The tetragonal crystal system is further separated into the hexagonal and trigonal or rhombohedral divisions. Under each crystal system the table lists by name the crystal classes which occur within that system.
The young guy looked at the stone and immediately saw a trigon on the girdle. Now...
Crystal System: Hexagonal (trigonal)
Habit: As well-formed, elongate, trigonal prisms, with smaller, second order prism faces on the corners. Prism faces are often striated parallel to direction of elongation (c axis).
Corundum crystallizes in the trigonal structure and is most often found as tabular hexagonal crystals (ruby) and hexagonal bipyramids (sapphire) although other habits may be encountered as well. Twinning is quite common.
Fig 311 Naturals at the girdle with trigons and line
These natural crystal faces are usually very small and lie immediately below the girdle. So long as they are not visible when looking at the crown they are ignored in cut-grading.
It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide). The typical shape of a Quartz crystal is a six-sided prism that ends in six-sided pyramids.
The Crystal structure comprises of hexagonal, trigonal and isometric lattice that transparent or semi-transparent in nature. The hardness of the stone is 9.25 with a Specific Gravity that ranges between 3.1 - 3.2.
The five other systems are hexagonal, trigonal, orthorhombic, monoclinic, and triclinic. Along with composition, crystal structure determines a mineral's properties.
The trigonal crystals range from translucent to transparent. Transparent calcite exhibits a double refraction effect (when you look through the crystal, singel items are doubled).
apatite, aquamarine, emerald
Trigonal systems
4 crystallographic axes, three of equal length, intersecting at 60 degree angles, the other three are longer or shorter and perpendicular to the first three
3 ...
hexagonal per Arem; hexagonal (trigonal) per Schumann
Variety:
green, yellow-green, brownish-yellow-green ...
5, system: trigonal and get a list of 8 mineral species with full data on each. I like to use these simple tests plus streak, and specific gravity when possible and use MinData to narrow down the options.
Uniaxial
Mineral having a unique direction of single refraction. One optic axis, tetragonal, hexagonal, and trigonal crystals are uniaxial ...
Quartz grows rapidly along its c-axis (three-fold or trigonal symmetry axis) direction and so never shows faces perpendicular to this direction.
The internal atomic structure of a crystal. All crystalline gems may be classified in one of seven groups according to the symmetry of their structure: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic.
In its purest form, corundum is colorless. It is a combination of aluminum and oxygen (Al2O3) and belongs to the trigonal crystal system. Non-gem varieties of corundum are primarily used as abrasives.
Contemporary production of CZ is virtually flawless, whereas diamond usually contains impurities and inclusions, or have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).
seven groups on the basis of specific set of angular, geometrical and symmetrical specifications. All crystalline gems can be classified in one of seven groups based on the above specifications. These groups are cubic, tetragonal, hexagonal, trigonal, ...
COLOR: various colors, except for red.
INDEX OF REFRACTION: 1.76 - 1.78
BIREFRINGENCE: 0.009
HARDNESS: 9
CLEAVAGE: none, although there is a rhombic parting
CRYSTAL SYSTEM: trigonal
PLEOCHROIC: strongly ...
Gemstones occurring in hexagonal crystals include beryl, corundum, quartz, and tourmaline. Some crystallographers further identify two subdivisions of hexagonal crystals: trigonal (corundum) and rhombohedral (quartz).
Tetragonal ...
Crystal structure affects mineral properties more than their chemical nature. Examples here include diamond (carbon, cubic) and graphite (carbon, hexagonal) and Calcite (trigonal) and aragonite (orthorhombic), both forms of calcium carbonate.
 See also: Crystal, Mineral, Color, Stone, Crystals
  
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