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"...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…

Cubes of Iron Pyrites frequently show a striation of their planes in one direction, which is perpendicular to the striation on all contiguous faces, owing to oscillatory combination with the pentagonal dodecahedron. This would not be possible in a holohedral cube.

Cube of Iron Pyrite

Cubes of Iron Pyrites frequently show a striation of their planes in one direction, which is perpendicular…

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.

This crystal of Aragonite shows a method of twinning where the twinning plane may change its direction, as when the two symmetrical faces of a prism successively serve as twinning planes.

Cyclic Twinning

This crystal of Aragonite shows a method of twinning where the twinning plane may change its direction,…

"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…

Crystal; This form is a combination of an icositetrahedron and a rhombic dodecahedron, where the faces of the icositetrahedron replace the trihedral angles of the dodecahedron. The parameters, m, of the icositetrahedron are less then two.

Dodecahedron

Crystal; This form is a combination of an icositetrahedron and a rhombic dodecahedron, where the faces…

Crystal; This form is a combination of an icositetrahedron and a rhombic dodecahedron, where the faces of the icositetrahedron replace the trihedral angles of the dodecahedron. The parameters, m, of the icositetrahedron are less then two.

Dodecahedron

Crystal; This form is a combination of an icositetrahedron and a rhombic dodecahedron, where the faces…

"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…

"This is the hemihedral form of the hexakis-octahedron and has the indicies {hkl}; it is bounded by twenty-four faces." -The Encyclopedia Britannica 1910

Dyakis-dodecahedron

"This is the hemihedral form of the hexakis-octahedron and has the indicies {hkl}; it is bounded by…

"...shows two crystals of tartaric acid, a a right-handed crystal of dextro-tartaric acid, and b a left-handed crystal of laevo-tartaric acid. The two crystals are enantimorphous." -The Encyclopedia Britannica 1910

Enantimorphous Crystals of Tartaric Acid

"...shows two crystals of tartaric acid, a a right-handed crystal of dextro-tartaric acid, and b a left-handed…

"Monoclinic. Crystals are often much elongated parallel to the ortho-axis with a prominent development of the faces of the orthodome zone, giving them a prismatic aspect. Striated parallel to the ortho-axis. Terminated usually at one end of the ortho-axis and most commonly by the two faces of a pyramid." &mdash; Ford, 1912

Epidote

"Monoclinic. Crystals are often much elongated parallel to the ortho-axis with a prominent development…

"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." &mdash; Hallock, 1905

First Right Square Octahedron

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

"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." &mdash; Hallock, 1905

First Right Square Prism

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

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." — Ford, 1912

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." — Ford, 1912

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination. Hexoctahedron observed at some times." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination. Hexoctahedron observed…

"Here the plane of projection is tangent to the sphere, and normals to all the faces are drawn fro mthe centre of the sphere to intersect the plane of projection." -The Encyclopedia Britannica 1910

Gnomonic Projection of a Cubic Crystal

"Here the plane of projection is tangent to the sphere, and normals to all the faces are drawn fro mthe…

Gold crystals of the form 3O3, {311}, sometimes resemble combinations of the rhombohedron and scalenohedron.

Gold

Gold crystals of the form 3O3, {311}, sometimes resemble combinations of the rhombohedron and scalenohedron.

"The second form of carbon, known as graphite, is a leaden-gray solid with a metallic luster and a specific gravity of 2 to 2.2, which crystallizes." &mdash;Hallock 1905

Graphite

"The second form of carbon, known as graphite, is a leaden-gray solid with a metallic luster and a specific…

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their planes developed at the expense of the other half, so as to produce a hexagonal scalenohedron.

Green Fluorspar from Saxony

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their…

Gypsum.

Gypsum

Gypsum.

Gypsum.

Gypsum

Gypsum.

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges beveled by prism and pyramid faces." &mdash; Ford, 1912

Gypsum

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges…

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges beveled by prism and pyramid faces. Other forms rare. Sometimes twinned." &mdash; Ford, 1912

Gypsum

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges…

This image represents a twin of the monoclinic mineral Gypsum. Its twinning plane is the orthopinacoid.

Gypsum (Twinning)

This image represents a twin of the monoclinic mineral Gypsum. Its twinning plane is the orthopinacoid.

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings. Edges of plates sometimes beveled with rhombohedral and pyramidal forms." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

Hemimorphism in the direction of the vertical axis has been observed on crystals of the tetragonal salt, Iodosuccinimide (C_4_H_4_O_2_NI, pictured). Its forms are (p), terminated at one end by (o) and (n), and at the other by (o) only.

Hemimorphism in the Direction of the Vertical Axis

Hemimorphism in the direction of the vertical axis has been observed on crystals of the tetragonal salt,…

"...bounded by twelve isosceles trianes each of which are equally inclined to two planes of symmetry." -The Encyclopedia Britannica 1910

Hexagonal Bipyramid

"...bounded by twelve isosceles trianes each of which are equally inclined to two planes of symmetry."…

"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." &mdash; Hallock, 1905

Hexagonal Prism

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

"Here each face of the octahedron is replaced by six scalene triangles, so that altogether there are fourty-eight faces. This is the greatest number of faces possible for an simple form in crystals." -The Encyclopedia Britannica 1910

Hexakis-octahedron

"Here each face of the octahedron is replaced by six scalene triangles, so that altogether there are…

"The hemihedral form {hkl} of the hexakis-octahedron; it is bounded by twenty-four scalene triangles and is the general form of the class." -The Encyclopedia Britannica 1910

Hexakis-tetrahedron

"The hemihedral form {hkl} of the hexakis-octahedron; it is bounded by twenty-four scalene triangles…

This form is bounded by forty-eight similar scalene triangles. Its solid angles are of three kinds: six octahedral, at the extremities of the principal axes; eight hexahedral, at the extremities of the trigonal axes; and twelve tetrahedral, at the extremities of the digonal axes. Naumann's symbol for this form is m 0 n; Dana's, m-n.

Hexoctahedron

This form is bounded by forty-eight similar scalene triangles. Its solid angles are of three kinds:…

This form shows the planes given on a crystal of hydrous nickel sulphate: the basal pinacoid, (c); three pyramids of the second order, (m), (o), and (q); two pyramids of the first order, (n) and (p); and the prisms of the first and second orders, (r) and (s).

Hydrous Nickel Sulphate

This form shows the planes given on a crystal of hydrous nickel sulphate: the basal pinacoid, (c); three…

This figure exhibits a combination occurring on the ferro-magnesian meta-silicate, Hypersthene. Forms: (a), (b), (m), (n), (h), (o), (c), (i), and (u).

Hypersthene

This figure exhibits a combination occurring on the ferro-magnesian meta-silicate, Hypersthene. Forms:…

"Bounded by twenty-four trapezoidal faces, and hence somethings called a 'trapezohedron.'" -The Encyclopedia Britannica 1910

Icositetrahedron

"Bounded by twenty-four trapezoidal faces, and hence somethings called a 'trapezohedron.'" -The Encyclopedia…

Also known as Stolzite, this crystal is bounded by the unit pyramid of the first order, P, {111} (o), and by the prism of the third order, (p).

Lead Tungstate

Also known as Stolzite, this crystal is bounded by the unit pyramid of the first order, P, {111} (o),…

Crystal; If the alternate planes of the hexoctahedron extend until the intersect, a new for will result. This form, the Pentagonal Icositetrahedron, is bounded by twenty-four similar but unsymmetrical sides. Naumann's symbol for this form is (m0n/2)r, y{lkh}; Miller's is (m0n/2)l, y{klh}; h>k>l. This form is known as 'left-handed' because it contains the left, top plane of the front, upper octant.

Lef-Handed Pentagonal Icositetrahedron

Crystal; If the alternate planes of the hexoctahedron extend until the intersect, a new for will result.…

"Isometric. Trapezohedral habit." &mdash; Ford, 1912

Leucite

"Isometric. Trapezohedral habit." — Ford, 1912

This is an illustration of crystals of the inner bark of the locust.

Locust

This is an illustration of crystals of the inner bark of the locust.

"This consists of four faces parallel to the brachy axis." -The Encyclopedia Britannica 1910

Macro-prism and Brachy-pinacoid

"This consists of four faces parallel to the brachy axis." -The Encyclopedia Britannica 1910

"Isometric. Octahedral habit, sometimes twinned octahedrons. Dodecahedron at times, either alone or with octahedron." &mdash; Ford, 1912

Magnetite

"Isometric. Octahedral habit, sometimes twinned octahedrons. Dodecahedron at times, either alone or…

As shown on this crystal of Magnetite, the octohedron, by the crowding out of two of its opposite faces, may become a rhombohedron.

Magnetite

As shown on this crystal of Magnetite, the octohedron, by the crowding out of two of its opposite faces,…

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example of malachite.

Malachite

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example…

"Five kinds of glass are made: flint glass, or crystal; crown glass, broad sheet glass, bottle, or green glass, and plate glass." &mdash;The Popular Cyclopedia, 1888

Glass manufacturing

"Five kinds of glass are made: flint glass, or crystal; crown glass, broad sheet glass, bottle, or green…