Competitive Exams: Geology Glossary N to R

Geology Glossary: N to R

  • Nicol Prism…Prism for producing polarized light, having split the light into two rays.
  • Nodule…A lump or node.
  • Opaque…Not transparent or translucent (you can't see through it).
  • Optic Axis…The direction of single refraction in a double refractive mineral.
  • Optic sign…If the lower index of the refractive reading does not vary when the gemstone is rotated, it is uniaxial and + optic sign, if the higher index is steady and the lower index varies the optical sign is-.
  • Ore… A mineral occurring in sufficient amounts to permit its recovery at a profit.
  • Organic… Compounds of carbon, i.e.: Coal, jet
  • Orient…Characteristic sheen and iridescence, as displayed in a good pearl. v Orientation… Applied to crystals, this means visualizing the disposition of the principal directions within the crystal.
  • Orogenesis…A complex phenomena that leads to the formation of mountain ranges.
  • Outcrop…Where the bed rock is exposed.
  • Paleontology… a division of geology that concerns itself with prehistoric life.
  • Paragenesis. The sequence in time in which a mineral crystallizes with respect to the other minerals.
  • Paramorph…A chemically identical crystal compared with the original, but the atoms have been rearranged so that they no longer conform to the original outline.
  • Parting…A smooth fracture that looks like a cleavage plane but takes place only on certain planes in the crystal, not between any set of atoms, like true cleavage.
  • Paste…Lead glass imitation stone.
  • Pegmatites____A very coarse platonic rock, generally granitic in composition. Usually forming dikes that cut granite or the gneisses and schists that border granite masses. They are coarse because the liquid residue at the time of their crystallization contained a high percentage of water and other volatile elements that did not go into the makeup of the common minerals of granite, and were concentrated in the residue.
  • Petrography. The branch of geology that deals with the description and classification of rocks.
  • Phaneritic rock. An igneous rock in which all of the essential minerals can be seen by the unaided eye.
  • Phenocryst…Crystal that is set in a finer grained ground mass.
  • Phosphorescence…Luminescence emanating from substances that have been irradiated with ultraviolet light or X-rays, but persisting after the source of stimulation has stopped.
  • Piezoelectric…A substance that becomes electrically charged by pressure.
  • Pipe…In geology, the tubular ascending structures in volcanic rock.
  • Placer…Concentrations of heavy or resistant minerals that have been transported to rivers or other water areas.
  • Plastics…Synthetic resin, hard, usually molded.
  • Pleochroism…Dichroism and similar effects shown by some biaxial minerals in which three distinct colors or shades can be seen by using a dichroscope (two at a time).
  • Plutonic rock… Rock that has solidified deep underground.
  • Pneumatolysis…The action of gases of a magma on the surrounding rocks and on the solidifying molten mass itself. Point… (diamonds). 1/100 of a carat, 01 ct.
  • Polarized Light…Light which vibrates in one direction or plane.
  • Polaroid…Class or plastic which passes light on one plane.
  • Potch… Native material in or around a gemstone, a dead spot or area. Ie: Ironstone surrounding opal.
  • Radioactive…Emitting alpha, beta, or gamma rays.
  • Reconstructed Stones…Old method of fusing Stones made from chips. Used now to describe amber remelted and fused from otherwise unusable pieces.
  • Reflection…The throwing off or back, light from the surface.
  • Refraction…The bending of light passing through one medium to another. The changing of direction.
  • Refractive Index…The measurement of the amount of change in direction of light passing through one medium to another. The ratio of the velocity of the light in air to the velocity of light in the medium being measured.
  • Refractometer…Instrument used to determine the refractive index of various substances.
  • Rock…Stoney matter. Any mass of mineral matter forming an essential part of the earth's crust.