Rhombohedral: Resembling a prism with six four-sided facets.
Ribbed: An undulating ridged texture, like a person's rib cage showing through the skin.
Rhombohedral:
Dolomite and ilmenite are the two most common minerals of this class.
Monoclinic: ...
Rhombohedral Division
hexagonal scalenohedron
trigonal trapezohedron
ditrigonal pyramid
rhombohedron
trigonal pyramid ...
A rhombohedral crystalline form with highly ordered structure. Formed from ice Ih by compressing it at temperature of 190-210 K. When heated it undergoes transformation to ice III.
Ice III ...
Rhombohedral
A rhombohedron (aka trigonal system) has a three-dimensional shape that is similar to a cube, but it has been skewed or inclined to one side making it oblique.
The Rhombohedral class has a 3-fold axis of rotoinversion, which is equivalent to a 3-fold axis of rotation and a center of symmetry. The general form is {hk-il} and the Hermann- Mauguin notation is -3.
Trigonal, rhombohedral, often tabular or often in massive forms.
Envronment:
Forms in coarse grained igneous rocks. Chiefly in nepheline - syenites ...
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.
Twin development.
Rhombohedral polysynthetic twin lamellae are seen in most specimens, inevitably accompanied by long white boehmite needles meeting at 86.1/93.9°.
Chabazite - orange rhombohedral crystals, confirmed by X-ray identification, were collected by D.O.T. and New York State Museum personnel during the 1991 excavation.
Chondrodite - scarce, unverified.
Blue smithsonite in rhombohedral crystals (Tsumeb Mineral Gallery page 224, Houston Museum of Natural Science)
Rose smithsonite colored red by the presence of cobalt. (Tsumeb Mineral Gallery, page 220, Pohl/Schloss Freudenstein collection) ...
Cleavage: good rhombohedral
Occurrence: world wide, intrusive igneous rocks Comments: collector item
Chalcanthite ... hydrated copper sulfate
ore of copper
Crystal sys: triclinic
Color: colorless
SG: 2.3
Hardness: 2.5 ...
Have perfect rhombohedral cleavage
3. Exhibit a very strong double refraction in transparent rhombohedrons caldera A vast depression at the top of a volcanic cone, ...
Topaz: A fluosilicate of aluminum that occurs in rhombohedral crystals and is used as a gemstone. Although it is a hard stone, topaz can be susceptible to breaking.
Crystal System: Hexagonal (rhombohedral)
Habit: Crystals commonly prismatic with rhombohedral terminations. Horizontal stria common on prism faces. Cryptocrystalline varieties are massive.
Hardness: 7
Fracture: Conchoidal ...
It crystallizes in the rhombohedral division of the hexagonal system, certain crystal forms being characteristic of the two varieties, ruby and sapphire.
It could be argued that since experiments were conducted with several flux agents like potassium hydroxide and barium fluoride,the final product, although described as rhombohedral ruby crystal, was actually red, violet, and bluish in color.
A glassy, colorless mineral occuring in rhombohedral crystals composed of beryllium silicate. From the Greek for imposter, deceiver, phenacite is a silicate of glucina, and receives its name from its deceptive similarity to quartz.
Durability -- Hardness 3. Difficult to facet due to perfect rhombohedral cleavage. Too soft for jewelry use. Massive forms are used for architectural and ornamental items, such as ashtrays, bookends, small sculptures, etc.
Hematite: Iron ore consisting of ferric oxide in crystalline form, hematite is silvery, shiny opaque stone that becomes a red powder when ground down. It manifests in splendent rhombohedral crystals that are very heavy and cold to the touch.
 See also: Crystal, Mineral, Crystals, Color, Rough
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