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Dallasite a name applied to the green and white jasper from Vancouver Island, British Columbia.


Danburite R.I. 1-63 --- 1-636; S.G. 3-00; H. 7; Rhombic; Colour, colourless and yellow; Localities, Madagascar, Japan, Burma and Switzerland.


Dark brown a classification of gem diamond.


Dark cape a classification of gem diamond. See Cape stones.


Datolite R.I. 1-625 --- 1- 669; S.G. 2-9 to 3-0; H. 5 to 5.5; Monoclinic; Colour, whitish, yellowish, colourless, reddish, greenish, brownish and mottled; Localities, U.S.A.


Deer horn the horn or antler of certain of the deer family has been used instead of ivory for small carvings, particularly for the netsukes of Japan. S.G. 1-6 to 1-85.


Delawarite a name applied to the aventurine feldspar found in Delaware Co., Pennsylvania, U.S.A.


Demantoid see Garnet.


Dendritic the tree or fern-like form assumed by some minerals, particularly when they are inclusions in others, such as the dark pigmenting minerals in quartz producing moss agates.


Density the comparison of the weight of a given volume of a substance with the weight of a similar volume of another substance used as a standard, see also Specific gravity,


Density correction tables see Chart


Derbyshire spar see Fluorspar and Blue John.


Deviation, minimum see Refractive index Chart.


Diakon see Perspex and Plastics, See Chart.


Diamante a term used in jewelry for paste set articles.


Diamantine trade name for ar abrasive made of aluminium oxide. Note, there is no diamond content.


Diamonair name applied to the synthetically produced yttrium aluminium garnet.


Diamond R.I. 2-417 to 2-420; S.G. 3-51 to 3-53; H. 10; Cubic, with a perfect cleavage parallel to the faces of the octahedron; Colour, colourless and pale tints of yellow, red, pink, green, blue, also brown. Some green diamonds have been artificially tinted by radium emanations. There are two distinct types of diamond;
(1) to which the majority of stones belong, which exhibits complete absorption beyond 3,000 A;
(2) the rarer "transparent" type, which transmits light down to 2,250 A. The dispersion (B-G)= 0 044; Occurrence is from alluvial deposits and from pipes (volcanic?) in South Africa; Localities, India, Brazil, South Africa, South West Africa, Tanzania, West Africa, Australia, Guyana, Congo, Ghana, Borneo and U.S.S.R. See Radiumtreated diamonds, Irradiated diamonds, Coloured diamonds, Industrial diamonds.


Diamondlite an American microscope used for the grading of diamonds.


Diamond colorimeter an American instrument for the colour grading of diamonds. The Diamolite is a similar instrument.


Diamond dust see Diamond powder.


Diamond gauge devices of assistance in the estimation of the weight of a mounted diamond. They are of two general types.
(a) A stencil gauge consisting of a thin sheet of metal or celluloid (the metal types often being in the form of folding leaves) in which are a series of differently sized circular apertures, each of which has a diameter agreeing with the diameter of a correctly fashioned diamond of given weight. The gauge is placed over the stone to be estimated and the aperture which just fits over the girdle of the stone gives the approximate weight of the diamond, each hole being marked with its value in carats or decimals of a carat.
(b) a pair of spring calipers with the moving arm fitted with a pointer which moves over a scale of numbers. The diameter of the stone is first measured and then the depth. The readings obtained are looked up in a book of tables supplied with the instrument and the approximate value read from them. It is considerably more accurate than the stencil gauge. See a]so Moe's diamond gauge.


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.


Diamond point the earliest form of diamond fashioning, being merely the polishing of the octahedral faces of the crystal to a regular shape.


Diamond powder boars and remainders from diamond cleaving and bruting reduced to powder by various mechanical methods (usually by crushing in an iron mortar) and then segregated into micron size groups by either air sifting, centrifuge, elutriation or settling in oil. Finest material commercially produced has a grit size of 1 micron (1 micron = O-OOl mm.). Diamond powder is used for grinding and polishing diamonds and other hard stones and for diamond saws and other industrial purposes. See Diamond saw.


Diamond saw a disc of phosphor bronze about 2" in diameter and about 07 to 12 millimeter in thickness, with a thickened periphery charged with diamond powder.


Diamondscope an American instrument consisting of a binocular microscope with a specially designed dark-field illuminator used for observing imperfections and internal features in diamonds and coloured stones.


Diamonds, Industrial certain types of diamonds, generally those unsuited for gemstone production, which have applications in modern industrial and engineering practice. They are used for:
......Diamond powder
......Glaziers diamonds
......Indenter hardness testers
......Truing tools for grinding wheels
......Turning tools,
......Wire drawing dies, etc.
......See also Ballas, Boart, Carbonardo, Diamond powder and Diamond saws.


Diaphaneity see Transparency.


Diasterism a star effect which is best seen when light is transmitted through the specimen. Phlogophite mica and some rose quartz shows this effect.


Diatomite a polishing powder produced from a soft silica composed of minute plant skeletons or diatoms. Sometimes known as "Fossil tripoli", it should not be confused with tripoli.


Dichroism the differential selective absorption of light seen in some doubly refractive stones. See Chart.


Dichroite see Iolite.


Dichroscope an instrument comprising a suitably cut rhomb of Iceland-spar and a lens system in a short tube, used for viewing the effects of dichroism.


Differential Selective absorption of light see Pleochroism, see Chart1


Diffraction Grating a series of fine lines ruled on glass or metal and used to produce spectra. They are used in some types of spectroscopes.


Diffraction of light the breaking up of white light into the spectrum colours when it passes through a narrow aperture. It is a special case of interference of light.


Diffusion column a tube containing two heavy liquids, one being less dense than the other, allowed to diffuse together so that the resultant liquid varies in density from top to bottom. Stones having specific gravities between the limits of the liquid will take up positions at differing levels. It is a method for quickly ascertaining the density of stones of slightly differing specific gravity such as stones of different colour of the same species.


Diffusion melt similar topux diffusion except the chemical compounds used are usually in separate layers and the crystallization occurs after diffusion of the chemicals when the mass is fused.


Dimethylaniline a liquid having an application in certain refractive index determinations. R.I.= 1 56.


Dimetric system alternative name for the Tetragonal system. Dimorphism; term applied to the case where two minerals have the same composition but a different crystal structure.


Dinny bone (Dinosaur bone) fossil dinosaur bone used as an ornamental stone.


Diopside R.I. 1-67 --- 1-70; S.G. 3-20 to 3-32; H. 5 to 6; Monoclinic; Colour, green; Localities, Italy and the U.S.A. Alternative names, Alalite and Malacolite. A massive dark violet-blue variety from Piedmont known as "Violane", is used as an ornamental stone.


Dioptase R.I. 1-655 --- 1-708; S.G. 3-3; H. 5; Trigonal; Colour, emerald-green; Localities, Siberia, Chile, Congo.


Dispersion the breaking up of white light into the spectrum colours when a ray passes across two inclined faces of the stone. In a gemstone it is known as "fire" See Chart 142.


Dodecahedron a geometrical solid having twelve faces. The rhombic dodecahedron has twelve lozenge-shaped faces and is a form found in the cubic system.


Dop
(1) Solder: A brass cup with a malleable copper stem filled with lead-tin solder o which the diamond is set in order to cut and polish the facets.
(2) Mechanical: A holder in which the diamond is held between steel claws tightened by a screw, enabling the stone to be automatically adjusted without re-setting.


Double refraction the effect caused by all crystals, except those of the cubic system, of splitting a ray of light which passes into them into two rays, which travel with differ


Double rose the name applied to a cut stone of spherical shape covered all over with triangular facets; it may be assumed to be two rose-cut stones base to base.


Doublets
(1) Composite stones,
(2) Spectroscopic, close pairs of lines seen in emission and absorption spectra.


Dravite Brown tourmaline.


Drop pearls pearls having a drop or pear-shape. Sometimes called "pear-eyes".


Dullam the concentrated gem gravel (illam) of Ceylon which contains the gem minerals.


Dumortierite R.I. 1678-1689 ; S.G. 3 26 to 3 36; H. 7; Rhombic; Colour, blue-violet; Locality, California, Arizona and Nevada.


Durability the resistance a stone possesses to forces which tend to destroy its lustre and polish. These forces may be physical or chemical.


Durangite R.I. 1- 66 --- 1 71; S.G. 3-97 to 4-07; H. 5; Monoclinic; Orange-red. From Mexico.


Dust, diamond see Diamond powder.


Dust diamond, assorted diamonds, usually 60 per carat down to 150 per carat in size, and having a degree of brilliancy so that they can be used for ornamentation of cheap jewelry without further polishing. A portion of this material is used industrially. Not to be confused with diamond dust.


Dwt, Pwt or pennyweight 24 grains, 0.05 ounces, metric 0.0648 grams.


Dyed stones a number of gem materials are sufliciently porous to readily take colouring agents and such treatment is commercially carried out. Chalcedony which is often inorganically stained various colours. Other stones sometimes dyed are turquoise jadeite, opal, serpentine, and alabaster. If the material has been stained and this can be proved, the fact should be disclosed, but in some cases this may not be possible, and in these cases, particularly the black and red colours of chalcedony, the names onyx, cornelian and sard would be commercially acceptable.