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"Hexagonal-rhombohedral. Common form is the simple rhombohedron, r, having nearly cubic angles. May show several different rhombohedrons." — Ford, 1912

Chabazite

"Hexagonal-rhombohedral. Common form is the simple rhombohedron, r, having nearly cubic angles. May…

This is a crystal of Copper Silicate (Dioptase) with a rhombohedron of the third order, (s), in combination with the prism of the second order, (m), and the negative rhombohedron, (r). This crystal is an example of rhombohedral tetartohedrism in the hexagonal system.

Copper Silicate Showing Rhombohedral Tetartohedrism In the Hexagonal System

This is a crystal of Copper Silicate (Dioptase) with a rhombohedron of the third order, (s), in combination…

"...represents a crystal of dioptase with the fundamental rhombohedron (r) and the hexagonal prism of the second order (m) combined with the rhombohedron (s)." -The Encyclopedia Britannica 1910

Crystal of Dioptase

"...represents a crystal of dioptase with the fundamental rhombohedron (r) and the hexagonal prism of…

This crystal is mainly terminated by a rhombohedron of the third order, (x). With this form are associated the prisms of the first and the second order, (m) and (a); another rhombohedron of the third order, (s), and the two rhombohedrons of the first order, (r).

Phenacite

This crystal is mainly terminated by a rhombohedron of the third order, (x). With this form are associated…

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid." — Ford, 1912

Quartz

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally…

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid. Sometimes one rhombohedron predominates or occurs alone." — Ford, 1912

Quartz

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally…

"Science has succeeded in classifying the thousands of known crystals in six systems, to each of which belongs a number of forms having some property in common. In order to classify these different crystals, the existence of certain lines within the crystal, called axes, is assumed, around which the form can be symmetrically build up. These axes are assumed to intersect in the center of the crystal, and to pass through from one side to the other." — Hallock, 1905

Rhombohedron

"Science has succeeded in classifying the thousands of known crystals in six systems, to each of which…

"The rhombohedron is a form consists of six rhombic-shaped faces, which correspond in their position to the alternate faces of a hexagonal pyramid of the first order. There may be two different orientations of rhombohedron. A negative rhombohedron is shown." — Ford, 1912

Negative rhombohedron

"The rhombohedron is a form consists of six rhombic-shaped faces, which correspond in their position…

"The rhombohedron is a form consists of six rhombic-shaped faces, which correspond in their position to the alternate faces of a hexagonal pyramid of the first order. There may be two different orientations of rhombohedron. A positive rhombohedron is shown." — Ford, 1912

Positive rhombohedron

"The rhombohedron is a form consists of six rhombic-shaped faces, which correspond in their position…

"The zig-zag lateral edges [of a scalenohedron] coincide with the similar edges of a rhombohedron..." -The Encyclopedia Britannica 1910

Scalenohedron with inscribed Rhombohedron

"The zig-zag lateral edges [of a scalenohedron] coincide with the similar edges of a rhombohedron..."…