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    Taaffeite R.I. 1-72; S.G. 3-60; H. 8; Hexagonal; Mauve; Ceylon; very rare.

    Table the name applied to the large central facet on the crown in the brilliant-cut and trap-cut stones.

    Talc the hydrated magnesium silicate of which soapstone is the massive variety. One of the softest minerals, it is the standard of 1 on the Mohs scale. See Steatite.

    Tanzanite the name given to the transparent blue zoisite.

    Tecali marble a green marble from Tecali, Pueblo, Mexico, which simulates jade.

    Tektite comprehensive name for moldavite and some other natural glasses.

    Television stone see Ulexite.

    Templet alternative name for the bezelfacet.

    Tetragonal (or Dimetric) one of the crystal systems. See Chart 112.

    Tetrahedron the crystal form bounded by four equilateral triangles and belonging to the cubic system (the tetrahedron is the geometrical solid with the smallest number of faces).

    Therm-luminescence a secondary light generated by certain substances when they are heated with invisible infra-red rays.

    Thomsonite R.I. 1-497--- 1-625; S.G. 2-3 to 2-4; H. 5 to 5.5 Rhombic; Colours, white, red, green and yellow (mottled); Locality, Lake Superior (West), U.S.A.

    Three-point see Diamond point.

    Thulite a variety of zoisite.

    Tiger's-eye a silica pseudomorph after crocidolite. See Quartz.

    Tin-cut the term applied to moulded glass imitation stones (pastes) which have had the facets polished on a lap.

    Tinstone alternative name for Cassiterite.

    "Titania" trade name for synthetic rutile.

    Titanite see Sphene.

    Toluene a hydrocarbon liquid used for the dilution of methylene iodide and bromoform in the preparation of heavy liquids; as an immersion liquid in certain refractive index tests and, owing to its low surface tension, in place of water in direct weighing method for determining density. R.I. = 1- 49; S.G. = 0-88.

    Topaz R.I. 1-607 --- 1-619 to 1-629 --- 1-637; S.G. 3-50 to 3-60; H. 8; Rhombic; Colours, yellow, blue, green and pink (generally heat treated); Localities, Brazil, Siberia, Ceylon, British Isles (rare) and U.S.A.

    Topaz, Brazilian name applied to the true yellow topaz. See Topuz.

    Topazolite yellow andradite. See Garnet.

    "Topaz, Oriental" a MISNOMER ( COMMERCIAL LABEL ) for yellow corundum.

    "Topaz, Scottish" a MISNOMER ( COMMERCIAL LABEL ) for yellow quartz.

    "Topez, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for synthetic corundum or spinel of a colour resembling a topaz colour. Mostly applied to the yellow.

    Tortoise-shell the horny shell of the carapace of the Hawks-bill Turtle (Chelone imbricata); R.I. 1-55 to 1-56; S.G. 1-26 to 1-35; Colours, mottled dark and light browns and yellows; Localities, Celebes, New Guinea, China, India, Africa and Australia. Tortoiseshell may be distinguished from "plastic', imitations by microscopic examination; the dark patches in the real material are seen to contain swarms of spherical reddish particles, whereas in the imitations the edges of the dark areas are more defined and lack the dot-like structure.

    Total reflection the name applied to the phenomenon which occurs when a ray of light travelling through a denser medium to a rarer medium at an angle greater than the critical angle suffers complete reflection back through the denser medium. See Critical angle of total reflection.

    Total reflectometers see Reflectometers.

    Touchstone black and unglazed Wedgwood pottery, or basanite, a velvet-black variety of quartz, used in the testing of precious metals by applying acids to the streaks made on the touchstone.

    Tourmalinated Quartz rock crystal with included crystals of, usually black, tourmaline.

    Tourmaline a complex boro-silicate; R.1. 1-616-1-634 to 1-63 --- 1-652; S.G. 3-00 to 3-15; H. 7 to 7.5; Trigonal; Colours, colourless (Achroite), red and pink (Rubellite), green, blue, yellow-green, honey-yellow, violet (Siberite), dark blue (Indicolite), brown (Dravite), black (Schorl); Localities, U.S.A., Ceylon, Madagascar, Germany, Brazil and Russia.

    "Tourmaline, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for synthetic spinel or corundum made in a colour to resemble tourmaline. It is generally the green colour to which the name is applied.

    Transparency or Diaphaneity the term used to describe the amount of light transmitted through a substance. Degrees of transparency are classed according to the amount of light which penetrates the substance:

    Transparent an object viewed through them shows outlines clear and distinct, e.g., most gemstones.
    1 Semi-transparent; the outlines of an object viewed through them would be blurred but a considerable amount of light can penetrate the stone.

    Translucent some light passes through but no object can be seen through the stone.
    1 Semi-translucent; light is only transmitted through the edges. Opaque; allows no light to pass through.

    "Transvaal jade" the light-green massive grossular garnet found in the Transvaal, South Africa, and used as a substitute for jade, has had this MISNOMER ( COMMERCIAL LABEL ) applied to it. Another MISNOMER ( COMMERCIAL LABEL ) for this material is South African jade. S.G. about 3-47 ; R.I. 1-73 ; H. 6.5.

    Trap-cut name applied to the style of cutting which consists of a table facet and a series of facets, each similarly disposed so that the contour of each runs parallel to that of the table. Also known as step cut. See Baguette-cut and Cuts of stones, Charts 181, 185.

    Trapeze the name applied to a stone cut in the form of a trapezoid (two parallel sides and two inclined). See Cuts of stones, Chart 182.

    Trapiche emerald an emerald crystal in which radially arranged albite separates clear emerald segments but producing a single crystal. The clear emerald parts are cut as gemstones.

    Treated opal black dyed opal.

    Tremolite Monoclinic; R.I. mean 1-61; S.G. 2-98; H. 5.5 to 6.5; An end member of the amphibole series of minerals, the other end member being actinolite (which see). The pale colours of nephrite are felted fibres of tremolite. The mineral also produces greenish chatoyant stones; (Canada), and a pink variety, called hexagonite (U.S.A.).

    Triangle the name applied to a trap-cut stone with an outline in the form of a triangle, generally an equilateral triangle. See Cuts of stones, Chart 182.

    Triboluminescence the phenomenon exhibited by certain minerals when they are rubbed or scratched, of showing a luminosity.

    Triclinic one of the crystal systems. See Chart 1 11.

    Trigonal or Rhombohedral system sub-division of the hexagonal system of crystallisation in which the principal axis is one of threefold instead of sixfold symmetry. Considered by some authorities as a separate system.

    Trimetric system alternative name for the rhombic system.

    Triplet a composite stone. See Charts.

    Triplex opals a composite stone consisting of an opal doublet (which see) which has a covering dome of rock crystal cemented over the face of the opal. It is actually a triplet.

    Tripoli a very fine-grained silica from Missouri and Oklahoma used as a polishing agent.

    Tugtupite a cyclamen-red ornamental mineral found in Greenland. It is near sodalite in composition and has a density near 2-36 and a refractive index of 1-50. The hardness is 6.

    "Tully" Memorial Medal; a medal awarded annually to the candidate submitting the best papers in the Pellowship Examination of the Gemmological Association of Great Britain which, in the opinion of the Council and Examiners, are of sufficiently high standard to merit the award.

    Tumbling a method used for the production of baroque shaped stones by churning them with abrasive and then with polishing compounds in a rotating drum. Used considerably in America.

    Turpentine an oil having a value in certain refractive index tests. R.I. = 1 47.

    Turquoise a basic phosphate of aluminium and some copper and iron; R.I. 1 61 to 1 65; S.G. 2 6 to 2 85; H. 6; Triclinic; Colours, blue and green; Localities, Persia, Egypt, Turkestan and U.S.A.

    Turquoise matrix Turquoise cut with some of the matrix it is found with, a brown limonite.

    Turquoise, Viennese see Viennese turquoise.

    Twin crystals two or more crystals of the same species which have intergrown together but always with reference to definite laws. They are often characterised by having reentrant angles, and are of three general types:
    (a) Contact twins; where two halves of a crystal are in reverse order, so that if one half is rotated through half a circle about the plane of joining (perpendicular to the twinning axis) the form of the normal crystal is obtained.
    (b) Interpenetrant twins; where two crystals have grown so that they penetrate one another, often producing cross and star forms.
    (c) Polysynthetic or repeated twins; are composed of a number of contact twins producing very thin plates, each crystal being arranged in reverse order to its neighbour. Sometimes called lamellar twinning.

    Two-point see Diamond point.