by Allan Taylor

Created on: June 13, 2010   Last Updated: March 16, 2011

Pablo Neruda, the Chilean Nobel Prize winning poet,  wrote in his poem  “Las Piedras del Cielo” about his love of  gems and minerals: 

                           Cuando se toca el topacio,  el topacio te toca

                          or, when you touch the topaz, the topaz touches you

A large topaz crystal  is a  wonderful thing to touch and handle, and look through.   My pet  coffee table crystal from Brazil measures 6 x 11 cm across the base and  8 cm  high,  bounded top and bottom by perfect basal cleavage planes.  It is super transparent and colorless,  heavy,  and feels cold to touch and lick.   What else can we learn from it?

The plentiful volume of flawless crystal would  allow the faceting of large colorless topaz gems,  which is sometimes done to simulate famous diamonds.  The refractive index of topaz  is moderately high at 1.63 to 1.64,   birefringence is 0.008,  specific gravity is  3.53  and hardness 8, making it the hardest of any silicate mineral.  Today, much colorless topaz is heat treated and irradiated to provide a ready supply of  magnificent sky-blue topaz which is a familiar gemstone found in jewelry shops.  Flawless  natural blue topaz having any depth of color is of  fairly rare occurrence.

Mineral collectors and gem hunters need to be familiar with the form and physical properties of topaz of which there are two main types viz.,  colorless and shades of blue topaz  often found in large crystals, sometimes several feet long,  in contrast to  the more valuable Brazilian Imperial Topaz,  which is yellow to orange color,  found as small prisms usually less than one inch in size.

Topaz crystals have orthorhombic symmetry.  Well  formed crystals are common and deserve study.  The four prism faces join to give the crystal a “lozenge” shape and it may be capped by a number of pyramid and dome faces,  where not truncated by the perfect basal cleavage plane.   The latter often provides a useful base to sit the crystal for display.

This perfect basal cleavage

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