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- Ugrandite series the name applied to the isomorphous series of garnets which contain the uvarovite/grossularite/andradite group.
- Ulexite (Mean) R.I. 1-51; S.G. 1-65 to 2-0; H. 1 to 2; Monoclinic (fibrous massive); Not a gemstone but of interest in that the fibres are so straight that when cut as thick plates with the fibres at right angles to the cut, print can be read through the stone and it is to this effect that the stone is called "Television stone".
- Ultra-violet light the invisible rays, in wave-length beyond the visible violet, having a range of between 1,000 and 3,800 Angstrom units. They are known best by the facility with which they affect a photographic plate and by the effect certain substances have of emitting a visible light when they are irradiated with ultra-violet rays (Fluorescence). Between 3,800 and 3,000 A the rays are usually known as long-wave ultra-violet light, and between 3,000 and 2,000 A as shortwave rays. Below about 2,000 A the radiations, for physical reasons, cannot be properly used.
- Ultra-violet lamps lamps used to produce ultra-violet light particularly for use in exciting luminescence in materials. They may be one of three types:
- (a) Electrically excited high pressure mercury vapour in a quartz glass tube with the powerful visible light filtered out with a suitable filter (Wood's glass filter). The main ultra-violet rays produced by such a lamp are about 3,650 A. This lamp is known as the long-wave lamp.
- (b) Electrically excited low pressure mercury vapour in a quartz, or special glass, envelope. The weak visible light and the long-wave ultra-violet rays are as far as possible filtered out by a suitable filter. Major radiation produced by this lamp is in the short-wave region at 2,537 A.
- (c) The fluorescent tube lamp. The lamp consists of a shortwave ultra-violet generator as in (b) but enclosed in an ordinary glass envelope upon the inside of which is sprayed a compound which "fluoresces" in the long-wave ultra-violet region. The lamp thus produces long-wave ultra-violet light with a range between 4,100A and 3,200 A so that the lamp emits further into the visible violet than does the high-pressure long-wave lamp. This lamp may be best for distinction between natural and synthetic emeralds.
- Unakite a granite-like rock consisting of a mixture of pink feldspar and green epidote and some quartz. The R.I. varies according to the mineral part from 1 52 for the feldspar, 1 55 for the quartz and 1 76 for the epidote. S.G. varies from 2 88 to 3 2. Mottled pink and green. U.S.A.
- Uniaxial the term used to describe the optical character of anisotropic crystals which have one direction of single refraction; confined to crystals of the tetragonal, trigonal and hexagonal crystal systems.
- Unit cell the unit of crystal structure which is the smallest part of a crystal which can still possess the characteristic properties of the crystal.
- Units of weights see Precious Mmetal or Gemstome Chart.
- Univalve molluscs pearl producing shellfish which have a shell in one piece, and not a shell in two halves as in the oysters and mussels. Such univalves are the abalone or Haliotidae and the giant conch.
- "Uralian emerald" a MISNOMER ( COMMERCIAL LABEL ) for the green demantoid garnet. See Garnet, Andradite.
- Utahlite alternative name for variscite.
- Uvarovite see Garnet.