Table
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The large, flat area at the top of a cut gemstone.
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Tachometer
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Instrument for measuring speed. In watch making, it refers to a timer or chronograph with a graduated dial on which speed can be read off in kilometres per hour or some other unit (see timer).
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Tahitian pearl
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These are dark-coloured pearls (also called black pearls). They are produced by the large, black-lipped pearl oyster Pinctada margaritifera (also called the Tahitian black pearl oyster), a mollusc found in the tropical Indo-Pacific Ocean. Black pearls come in many colours, including many body shades and overtone tints including grey (light grey to almost black), peacock green (especially valuable), aubergine (eggplant), and deep brown. The colour of the dark nacre is determined by the minerals in the oyster's diet (plankton) and in its environment. Many "black pearls" are dyed or irradiated to enhance or change their colour; it is difficult to tell a natural pearl from a treated pearl.
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Tanzanite
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A valuable, transparent, blue-violet type of zoisite resembling sapphire. It is often heat-treated in order to produce a deeper blue-violet colour.
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Termineur
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French term for an independent watchmaker (or workshop) engaged in assembling watches, either wholly or in part, for the account of an "établisseur" or a "manufacture", who supply the necessary loose parts.
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Tiger’s eye
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A yellowish-brown to reddish-brown gemstone that has a silky lustre. The gemstone has bands of yellow and brown; when viewed from the opposite direction, the colours are reversed. Usually set as a cabochon.
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Timer
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Instrument used for registering intervals of time (durations, brief times), without any indication of the time of day.
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Titanium
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A metallic element that has been used mainly in industry because of its lightness, strength and high melting point, but has in recent years been used in some jewellery, owing to the attractive range of colours that it acquires by being heated.
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Tonneau
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The shape of a watch-case like the cross section of a barrel with bulging sides and extending lugs.
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Tourbillon
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Device invented to eliminate errors of rate in the vertical positions. It consists of a mobile carriage or cage carrying all the parts of the escapement, with the balance in the centre. The escape pinion turns about the fixed fourth wheel. The case makes one revolution per minute, thus annulling errors of rate in the vertical positions.
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Tourmaline
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A dichroic gemstone that comes in many, many different colours; it also appears to have different colours depending on the angle at which it is seen. Tourmaline has the greatest colour range of any gemstone - the lighter colours are more valuable than the darker colours. It ranges in colour from pink to green to red (rubellite) to purple to blue-green (indicolite) to colourless (achroite) to black. Watermelon tourmaline is both pink and green. Tourmaline occurs as an elongate three-sided prism and is mined in Brazil, The Ural mountains in Russia, Namibia, Sri Lanka, and California.
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Train
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A specific set of wheels and pinions in a watch movement.
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Translucent
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Semi-transparent.
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Triple Complication
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A minute-repeater combined with a perpetual calendar and chronograph in a single watch.
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Turquoise
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A non-translucent, porous semi-precious stone that is usually cut as a cabochon.
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