"Orthorhombic. Crystals resemble closely those of barite. Commonly tabular parallel to the base or prismatic parallel to the brachy- or macro-axis with prominent development of the domes. Crystals which are elongated parallel to the brachy-axis are frequently terminated in front by four faces in nearly equal development, consisting of 2 prism faces and 2 of the macrodome." — Ford, 1912

Celestite

"Orthorhombic. Crystals resemble closely those of barite. Commonly tabular parallel to the base or prismatic…

"A crystal has a center of symmetry if an imaginary line is passed from some point on its surface through its center, and a similar point is found on the line at an equal distance beyond the center." — Ford, 1912

Symmetry center

"A crystal has a center of symmetry if an imaginary line is passed from some point on its surface through…

"Habit varied and crystal show many forms. Crystals often tabular parallel to brachypinacoid." — Ford, 1912

Cerussite

"Habit varied and crystal show many forms. Crystals often tabular parallel to brachypinacoid." —…

Chabazite.

Chabazite

Chabazite.

"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…

Chalcanthite.

Chalcanthite

Chalcanthite.

"Orthorhombic. Usually in small tabular crystals with hexagonal outline. Striated parallel to the brachy-axis. Often twinned in pseudohexagonal forms." — Ford, 1912

Chalcocite

"Orthorhombic. Usually in small tabular crystals with hexagonal outline. Striated parallel to the brachy-axis.…

"A drawing of a crystal showing a combination of the cube, octahedron and rhombic dodecahedron is shown, in which the faces are lettered the same as the corresponding poles in the projection." -The Encyclopedia Britannica 1910

Clinographic Drawing of a Cubic Crystal

"A drawing of a crystal showing a combination of the cube, octahedron and rhombic dodecahedron is shown,…

"Orthohombic. Habit of crystals is short prismatic; often in square prisms because of prominent development of the vertical pinacoids. Terminated by basal plane, pyramids and domes; frequently complex." — Ford, 1912

Columbite

"Orthohombic. Habit of crystals is short prismatic; often in square prisms because of prominent development…

"The combination of hexakis-octahedron and cube.

Combination of Hexakis-octahedron and Cube

"The combination of hexakis-octahedron and cube.

The combination of icositetrahedron and rhombic dodecahedron.

Combination of Icositetrahedron and Rhombic Dodecahedron

The combination of icositetrahedron and rhombic dodecahedron.

The combination of an icositetrahedron and a cube.

Combination of Icositetrahedron and Cube

The combination of an icositetrahedron and a cube.

The combination of an icositetrahedron and an octahedron.

Combination of Icosietetrahedron and Octahedron

The combination of an icositetrahedron and an octahedron.

Represents the combination of pentagonal dodcahedron, dyakis-dodecahedron and octahedron.

Combination of Pentagonal Dodecahedron, Dyakis-dodecahedron, and Octahedron

Represents the combination of pentagonal dodcahedron, dyakis-dodecahedron and octahedron.

"...shows the rhombic dodecahedron in combination with the octahedron." -The Encyclopedia Britannica 1910

Combination of Rhombic Dodecahedron and Octahedron

"...shows the rhombic dodecahedron in combination with the octahedron." -The Encyclopedia Britannica…

The combination of tetrakis-hexahedron and cube.

Combination of Tetrakis-hexahedron and Cube

The combination of tetrakis-hexahedron and cube.

The combination of a cube and a triakis-octahedron.

Combination of Triakis-octahedron and cube

The combination of a cube and a triakis-octahedron.

"...a combination of these two tetrahedra, and represents a crystal of blende, in which the four larger faces are dull and striated, whilst the four smaller are bright and smooth." -The Encyclopedia Britannica 1910

Combination of two Tetrahedra

"...a combination of these two tetrahedra, and represents a crystal of blende, in which the four larger…

Represents one way a tetrahedron and a cube might combine.

Combination of Tetrahedron and Cube

Represents one way a tetrahedron and a cube might combine.

Corundum.

Corundum

Corundum.

Corundum.

Corundum

Corundum.

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…

A crystal cross set in enameled gold created by a German craftsman in the 16th century.

Cross

A crystal cross set in enameled gold created by a German craftsman in the 16th century.

An illustration of a crystal showing triangular markings.

Crystal

An illustration of a crystal showing triangular markings.

Natural Crystal. 1, Fluorite.

Crystal

Natural Crystal. 1, Fluorite.

"...represents a crystal of augite bounded by th clino-pinacoid, the ortho-pinacoid, a prism, and a hemi-pyramid." -The Encyclopedia Britannica 1910

Crystal of Augite

"...represents a crystal of augite bounded by th clino-pinacoid, the ortho-pinacoid, a prism, and a…

"Here there is only a [center] of symmetry. All the forms are pina-coids, each consisting of only two paarallel faces." -The Encyclopedia Britannica 1910

Crystal of Axinite

"Here there is only a [center] of symmetry. All the forms are pina-coids, each consisting of only two…

"...a combination of two bisphenoids (P and P'), two bipyramids of the second order (b and c), and the basal pinacoid (a)." -The Encyclopedia Britannica 1910

Crystal of Chalcopyrite

"...a combination of two bisphenoids (P and P'), two bipyramids of the second order (b and c), and the…

"...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…

"...a combination of tetragonal prism of the first order with a tetragonal bipyramid of the third order and the basal pinacoid." -The Encyclopedia Britannica 1910

Crystal of Fergusonite

"...a combination of tetragonal prism of the first order with a tetragonal bipyramid of the third order…

"...a combination of the brachy-pinacoid and a prism, with the pedion, two brachy-domes and two marco-domes at the upper end, and a pyramid at the lower end." -The Encyclopedia Britannica 1910

Crystal of Hemmorphite

"...a combination of the brachy-pinacoid and a prism, with the pedion, two brachy-domes and two marco-domes…

Holohedral orthorhombic combination

Crystal of Hypersthene

Holohedral orthorhombic combination

"...represents a crystal of tourmaline wit the trigonal prism, hexagonal prism, and a trigonal pyramid at each end." -The Encyclopedia Britannica 1910

Crystal of Tourmaline

"...represents a crystal of tourmaline wit the trigonal prism, hexagonal prism, and a trigonal pyramid…

"...shows the actual shape of a crystal of ziron from Ceylon" -The Encyclopedia Britannica 1910

Crystal of Zircon (actual)

"...shows the actual shape of a crystal of ziron from Ceylon" -The Encyclopedia Britannica 1910

"...the ideally developed form [of a zircon crystal]. The crystal is a combination of five simple forms: two tetragonal prisms, two tetragonal bipyramids, and one ditetragonal bipyramid." -The Encyclopedia Britannica 1910

Crystal of Zircon (ideal)

"...the ideally developed form [of a zircon crystal]. The crystal is a combination of five simple forms:…

A beryl crystal, an example of a basal pinacoid.

Beryl crystal

A beryl crystal, an example of a basal pinacoid.

A beryl crystal, an example of a basal pinacoid.

Beryl crystal

A beryl crystal, an example of a basal pinacoid.

A beryl crystal, an example of a basal pinacoid.

Beryl crystal

A beryl crystal, an example of a basal pinacoid.

A beryl crystal, an example of a basal pinacoid.

Beryl crystal

A beryl crystal, an example of a basal pinacoid.

"The forms present upon it are two pyramids of different slope but each intersecting all three of the crystal axes when properly extended. The lower pyramid intersects the two horizontal axes at distances which are proportional to their unit lengths and if it was extended as shown by the dotted lines would also cut the vertical axis at a distance proportional to its unit length." — Ford, 1912

Sulphur crystal

"The forms present upon it are two pyramids of different slope but each intersecting all three of the…

"...take any three edges formed by the intersection of three faces of a crystal. These axes are called the crystallographic axes, and the planes in which they lie are the axial planes. A fourth face on the crystal intersecting these three axes in the points A, B, C is taken as the parametral plan, and the lengths OA:OB:OC are the parameters of the crystal." -The Encyclopedia Britannica 1910

Crystallographic Axes of Reference

"...take any three edges formed by the intersection of three faces of a crystal. These axes are called…

Forms of crystals.

Crystals, Forms of

Forms of crystals.

"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

Cube

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

Represents the combination of an octrahedron and a cube, with the cube faces predominate.

Cube in Combination with Octahedron

Represents the combination of an octrahedron and a cube, with the cube faces predominate.

Represents the combination of a cube and an octahedron, with both faces being equal.

Cubo-octahedron

Represents the combination of a cube and an octahedron, with both faces being equal.

"Monoclinic. Habit varied. Crystals usually nearly equidimensional in the three axial directions and often complex in development." — Ford, 1912

Datolite

"Monoclinic. Habit varied. Crystals usually nearly equidimensional in the three axial directions and…

"This is the hemihedral form of the triakis-octahedron; it has the indices {hhk} and is bounded by tweleve trapazoidal faces." -The Encyclopedia Britannica 1910

Deltoid Dodecahedron

"This is the hemihedral form of the triakis-octahedron; it has the indices {hhk} and is bounded by tweleve…

"Curved faces of the hexoctahedron are frequently observed." — Ford, 1912

Diamond

"Curved faces of the hexoctahedron are frequently observed." — Ford, 1912

Diamond.

Diamond

Diamond.

"One crystalline form of diamond." —Hallock 1905

Diamond

"One crystalline form of diamond." —Hallock 1905

"One crystalline form of diamond." —Hallock 1905

Diamond

"One crystalline form of diamond." —Hallock 1905

"One crystalline form of diamond." —Hallock 1905

Diamond

"One crystalline form of diamond." —Hallock 1905

"One crystalline form of diamond." —Hallock 1905

Diamond

"One crystalline form of diamond." —Hallock 1905

The diamond is cut to show the stone's brilliance, creating different parts to the cut: table, crown, girdle, pavilion, culet.

Diamond Cut

The diamond is cut to show the stone's brilliance, creating different parts to the cut: table, crown,…

Various types of square cut diamonds. Top row: Old square cut diamonds. Bottom row: English square cut diamonds.

Square Cut Diamonds

Various types of square cut diamonds. Top row: Old square cut diamonds. Bottom row: English square cut…

"...bounded by twenty-four scalene triangles...This form may be considered as a combination of two scalenohedra, a direct and an inverse." -The Encyclopedia Britannica 1910

Dihexagonal Bipyramid

"...bounded by twenty-four scalene triangles...This form may be considered as a combination of two scalenohedra,…

Diopside is a monoclinic pyroxene mineral with composition MgCaSi<small><sub>2</sub></small>O<small><sub>6</sub></small>. It forms complete solid solution series with hedenbergite (FeCaSi<small><sub>2</sub></small>O<small><sub>6</sub></small>) and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93&deg; typical of the pyroxene series. It has a Mohs hardness of six and a Vickers hardness of 7.7.

Diopside

Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution…

"Hexagonal-rhombohedral. Crystals are usually the unit rhombohedron (cleavage rhombohedron). Faces often curved, and sometimes so acurately as to form "saddle-shaped" crystals. Other forms rare." &mdash; Ford, 1912

Dolomite

"Hexagonal-rhombohedral. Crystals are usually the unit rhombohedron (cleavage rhombohedron). Faces often…