Some  Topaz Colour Enhancements
Cobalt-diffused topaz along with treated colour and surface-coated topaz.  The picture shows three typical treated blue topaz, and one surface coated topaz ie the specimen which has an iridescent appearance.  The two types mentioned are easily identified on a  refractometer .  The bluish green stone in the centre is cobalt diffused, which can present a gemmological problem for identification with a standard refractometer.
 

Four Cobalt-Diffused Topaz
All the specimens observed produce a shadow column on the refractometer up to the liquid limit of RI 1.81.   The bluer stones do show a shadow  system at approx RI  1.61, 1.62,  ( within the overall scale shadow column) and the double refraction product approx.0.010.
The greener specimens (Courtesy SAN DIEGO LAB, THOM UNDERWOOD GG ISA), give no inkling of their topaz identity on a refractometer.  When seen at Tucson, February 1998, Dr. W Hanneman was present, and his "Jeweler's Eye" reflectivity meter registered all four stones in the refractive area of  cubic zirconia.  This confirmed the refractometer's indication of their high refraction above 1.81.
 The non-refractometer reading in this case stemmed from the cobalt-diffused surface, which is evidently altered (possibly to a distinct mineral surface).
Without knowing the identity, I was challenged by Hanneman and others to identify the stones.  The only hint given was that they were a good "Paraiba colour", in allusion to the distinct shades of blue and green seen in the tourmalines from that Brazilian locality.  It should be said that the large size and jumbled juxtaposition threw doubt on such an identity, especially when a fine one carat Paraiba specimen can command $4000 per carat in the trade.

A penlight torch was produced, and I sat on the bed of the motel room and adopted a "Visual Optics" stance.  My comments were as follows- "The refraction is low, there is double refraction, but not enough for tourmaline, and the B/D ratio is less than that of tourmaline".  Without a scale I was forced to approximate that,  "The RI is about 1.5 or 1.6, and the DR approx 0.10, which suggests  an identity in the region of beryl or topaz.".    The small audience was visibly impressed.  The identity was then revealed as topaz.

The extraordinary fact for me was that  here was a gemstone which indicated its RI  above 1.81 on a refractometer, and the suggestion of CZ on a reflectivity meter.   In spite of the instrumental suggestions, The Hodgkinson method literally saw  through the subterfuge to recognise the innate optical properties which come from the body of the stone.  The refractometer can, of course, only respond to the surface of a gemstone.

Stippled Appearance
A view of the diffused topaz immersed in baby oil picks out the unnatural stippled nature of the surface.  For those familiar with the slippery feel of topaz, this was decidedly lacking