16 March 2002
Summary of Gemstone Faceting and Crystals
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Introduction
This explores typical gemstone cuts and faceting, as well as providing a tabular presentation of the relationship of many common gemstones to crystal systems and classes. The Annex also shows the common association of gemstones with the chakra system.
Technical comment
The proportions of a stone, as well as its polish and precision of faceting, determine how much of the diamond's potential fire and beauty may be released. The way a diamond is cut profoundly influence its sparkle, fire and brilliance, as well as its perceived size and even, to some degree its apparent color. In order to maximize the diamond's brilliance it must be well polished and cut in a geometrically precise manner. This means properly aligning the facets so light will enter the diamond and reflect back through the large top facet, or table of the diamond.
As a general rule only translucent, semi-translucent or clear gemstones are cut with facets. There are several basic cuts:
- cabochon (flat on the bottom and rounded on the top): the oldest and most common cut. Variants: round, oval, square, triangular, and even octagonal cuts.
- round brilliant cut: the most popular faceted cut for most stones,
including diamonds, refracting the most light. A full cut is around brilliant
stone with 57 or 58 facets (32 on the crown, 24 onthe pavilion, the table
counts as one and the culet counts as one), wheras a single cut is a round
brilliant cut with 17 facets (a round shaped diamond with 18 facets, 8 on
the crown, 8 on the pavilion, with the table counts as one and the culet counts
as the other one). A standard round brilliant cut usually has 24 facets on
the pavilion, 32 facets on the crown, a table facet, and may or may not have
a culet facet. In addition to the usual number of facets the girdle may be
faceted.
- Lower-girdle facets: one of 16 facets found on the pavilion of the roundbrilliant cut diamond. The top section of the lower-girdle facet forms lower girdle border.
- Upper-girdle facets: one of 16 facets found on the lower crown section ofround brilliant cut diamond. The lower section of the facet forms thegirdle border and the facet extends approx. half way up the crown.
- Pavillion facet: one of eight facets found on the pavilion of the round brilliantcut diamond. Each pavilion facet comes to a point where it reaches thegirdle.
- Star facet: One of eight facets found on the upper crown section of the diamond. The top horizontal section of the facet forms the table border and the facet extends approx. half-way down the crown.
- oval cut: a modification of the round brilliant cut.
- emerald cut: less refractive but very popular for cutting emeralds. Variants: square and trillion. A diamond that has a step cutting arrangement.It is square or rectangular with parallel rows of elongated narrowstep-up facets on the crown and pavilion. An emerald cut diamond hascorner facets at each one of the four corners.
- marquise cut: almost exclusively used with diamonds.
- radiant cut: 70 facets, is rectangular in shape and has cut corners. The crown section has a combination brilliant-step cut arrangement and the pavilion section has more of a brilliant styling with a slight step-styling in the upper sections just below the girdle
There are many other types of cuts of gemstones, cushion top, rose cut, pear cut, fancy cut, etc. These cuts are relatively rare compared to the six cuts described above. Bastard-cut a term used for fashioned stones which do not conform to the recognised typical forms, or which show some slight modification from the "pure" forms. The term applies only to those stones which have a regular and symmetrical arrangement of the facets; should they be irregular or haphazard the term Cap-cut is used.
There is no universal agreement on what angles to use for faceting various gemstones. Each set of faceting angles from generally accepted sources may well have slightly different optical characteristics. The definition of which is "best" is purely a matter of personal preference.
Why a diamond is cut that way?
- Bezel that part of a cut stone which lies above the girdle or setting edge. In the brilliant-cut stone it has the table facet and 32 surrounding facets. An alternative name is Crown.
- Girdle name applied to the outer edge of a cut stone. It is the line of junction of the top (crown) and the base (pavilion). Also termed the setting edge.
- Diamond point the relation of the table of a cut diamond to the underlying regular octahedron. It is said to be four-point, if the table be cut parallel to the face of the cube, that is across the corner of the octahedron so that the resulting section is square; three-point, if the table be parallel to an octahedral face; and two-point, if the table be parallel to the face of the rhombic dodecahedron and therefore to an edge of the octahedron, while equally inclined to its two faces meeting in that edge.
- Eight-cut a simple modification of the brilliant-cut used for small diamonds. The table being surrounded by eight foursided faces. Eight Cut or Single Cut Often used for very small diamonds. These only have eight four-sided facets on the crown, eight on the pavilion, plus the table and culet, making 18 in total.
- Swiss Cut is halfway between a brilliant and an eight cut, with 34 facets in total.
- Pinacoidal face a crystal face that is parallel to two of the crystal axes.
Facets
Each facet of a brilliant has a name of its own: skew, skill, bezel, quoin, etc. facet angles [more]
- Table (facet): the name applied to the large central facet on the crown in the brilliant-cut and trap-cut stones.
- Star facets: eight triangular facets which surround the table in the brilliant cut.
- Kites: name sometimes applied to the eight crown facets known as
the bezels and quoins:
- Quoin facets: four facets adjacent to the bezel facets, on the crown in a brilliant-cut stone. These eight facets, the quoins and bezels are often called collectively the bezels or kites.
- Bezel facets (or Templet): eight large four-sided facets touching the table (hence the French expression coin de table, hence the association with 'quoin') in the crown of a brilliant-cut stone.
- Break facets: the 16 small triangular facets on the crown and edging
the girdle, and the 16 similar facets on the pavilion. They are the cross
and skill facets. The combined 16 facets, the eight cross (haléfi
in French) and the eight skill (haléfi de culasse), are often
collectively known as break facets, half facets or halves (hence haléfi).
- Cross facets (skew facets): eight of the small three-sided facets around the girdle edge on the crown, which in the case of a modern circular stone have the same size and shape as the eight skill facets adjacent. In older oval-shaped stones these facets are the eight larger of the 16 edge facets.
- Skill facets: eight small triangular facets around the girdle edge of the crown in the brilliant cut. They are adjacent to the cross or skew facets.
- Pavilion facets: the long five-sided facets (coin de culasse in French) on the pavilion, or base, of a brilliant-cut stone.
- Culet (facet): the small facet at the base of the pavilion of a brilliantcut stone parallel to the table facet. Its main function is to prevent splintering but it is often omitted in modern cut stones, but it may be open. Also spelt collet and culette.
Crystals and gemstones
| Crystal system |
Crystallographic axes | Symmetry | Class | Examples | Chakras | |||||
| Number | Angle | Length | Axes | Planes | Centre | Subdues | Opens | |||
| Cubic Isometric |
3 | right angles | equal | 13 (6 two-fold, 4 three-fold, 3 four-fold) |
9 | 1 | element | diamond | 7 | |
| silicate | garnet (almandine, andradite, grossular, pyrope, spessartine, uvarovite) | 1 | 1 | |||||||
| oxide | spinel | |||||||||
| oxide | periclase | |||||||||
| Tetragonal | 3 | right angles | 2 horizontal axes of equal length; vertical axis is either longer or shorter. | 5 (4 two-fold, 1 four-fold) |
5 | 1 | silicate | zircon | ||
| silicate | scapolite | |||||||||
| Hexagonal | 4 | 3 horizontal axes of equal length and at 60 degrees to each other. | vertical axis is usually longer and at right angles to the horizontal axes. | 7 (6 two-fold, 1 six-fold) |
7 | 1 | silicate | beryl (emerald) | 4 | 4 |
| silicate | beryl (aquamarine) | 5 | 5 | |||||||
| silicate | benitoite | 6 | ||||||||
| silicate | sugilite | 6 | ||||||||
| phosphate | apatite | |||||||||
| oxide | taaffeite | |||||||||
| Trigonal | 4 | 3 horizontal axes of equal length and at 60 degree angles to each other. | vertical axis is usually longer and at right angles to the horizontal axes. | 4 (3 two-fold, 1 three-fold) |
3 | 1 | oxide | corundum (ruby) | 1 | 1, 2 |
| oxide | corundum (sapphire) | 5 | 6 | |||||||
| silicate | tourmaline (buergerite, dravite, elbaite, schorl, uvite) | 1 | 3 | |||||||
| carbonate | rhodochrosite | 4 | ||||||||
| carbonate | calcite | 3 | ||||||||
| silicate | quartz (clear) | 2 | 7 | |||||||
| silicate | quartz (amethyst) | 6 | ||||||||
| silicate | quartz (rose) | 4 | ||||||||
| silicate | quartz (agate) | 1 | 2 | |||||||
| silicate | quartz (citrine) |
|
||||||||
| Orthorhombic | 3 | all at right angles to each other. | all of unequal length | 3 (all two-fold). |
3 | 1 | oxide | chrysoberyl (alexandrite) | ||
| silicate | olivine (peridot) | 3 | 4 | |||||||
| carbonate | sinhalite | |||||||||
| silicate | andalusite | |||||||||
| silicate | kornerupine | |||||||||
| silicate | cordierite | |||||||||
| silicate | zoisite (tanzanite) | |||||||||
| silicate | topaz | 5 | ||||||||
| Monoclinic | 3 | 2 axes are inclined at an angle other than 90 degrees | third axis is at right angles to the other two. | 1 (two-fold). |
1 | 1 | ||||
| silicate | jade | 2 | 4 | |||||||
| silicate | euclase | |||||||||
| silicate | diopside | |||||||||
| silicate | spodumene (kunzite) | 4 | ||||||||
| silicate | chrysocolla | 2 | ||||||||
| silicate | titanite (sphene) | |||||||||
| carbonate | azurite | 5 | ||||||||
| phosphate | brazilianite | |||||||||
| Triclinic | 3 | all of unequal length (least symmetrical of all crystal systems) | all inclined at angles other than 90 degrees to each other. | 0 | 0 | 1 | silicate | microcline (amazonite, perthite) | 5 | |
| silicate | rhodonite | |||||||||
| silicate | kyanite | 5 | ||||||||
| silicate | axinite | |||||||||
| silicate | oligoclase (sunstone) | |||||||||
| silicate | oligoclase (moonstone) | 3 | 6 | |||||||
| phosphate | turquoise | 5 | 5 | |||||||
| (Amorphous) | -- | -- | -- | -- | -- | -- | mineraloid | amber | 3 | 2 |
| opal | 7 | |||||||||
| tektites | ||||||||||
Classification of gemstones
It was only in this century that stones were classified into various categories such as precious stones, semiprecious stones and ornamental stones. In 1880, for example, the diamond, corundum, ruby, emerald, sapphire, amethyst, agate, aventurine, garnet, lapis lazuli, opal, topaz and turquoise were all classified as precious stones. Well into the 20th century the preference was to divide the minerals used in jewelry into two categories, gems (when mounted in jewelry) and ornamental stones (carvings, mosaics, inlays, etc); the term precious stone is strictly reserved for diamonds, rubies, sapphires and emeralds. Modern classification separates minerals into several categories [more]: pure elements; sulphides; halides; oxides; carbonates; phosphates; silicates; non-crystalline and organic materials
Classification of gemstones http://www.tradeshop.com/gems/classify.html
Indian classification of gems and jewels http://www.urday.com/gemo2.htm
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