Re: how slow have we made the speed of light ?

If you ask what (man-made) materials have the largest value of index
of refraction.


I think it may be moissanite.It began to make a huge splash in the gem 
world as the latest and greatest diamond simulant in mid-1998 . This 
synthetic silicon carbide crystal was discovered by a materials scientist 
at North Carolina State University and is being developed by Charles & 
Colvard (formerly known as C3) and Cree Research. Its natural counterpart 
has been found only in meteorites and is deep green, but the synthetic 
moissanite is near colorless and will probably continue fool quite a few 
jewelers and jewelry buyers. Its high refractive index and extreme 
hardness (second only to diamond) make it an effective diamond simulant, 
and the popular diamond probes that test thermal conductivity fail to 
distinguish it from diamond. Charles & Colvard is also marketing a 
detector that can distinguish between moissanite and diamond. Another way 
to identify it is by its double refraction -- look through the stone from 
several different angles and you should see double images of facet lines 
or objects viewed through the stone.

Chemical composition -- Silicon carbide. 

Color -- Near colorless, usually slightly greenish or grayish. 

Optics -- R.I. 2.65-2.69. Uniaxial. 

Durability -- Hardness 9.25-9.5 (second in hardness to diamond). 

Crystal structure -- Hexagonal. 

Specific Gravity-- 3.22. 

Moissanite, which has been found naturally in tiny amounts in meteorites, 
is the most recently touted manmade diamond imitation. Developed from 
material produced by Cree Research, C3 Inc. began to market it through 
selected retailers in mid-1998. In October 1999, C3 announced that they 
would change their business name to Charles & Colvard. If properly 
oriented and well cut, moissanite strongly resembles diamond, due to its 
high refractive index and dispersion. As it has similar thermal 
conductivity to diamond, moissanite can pass as diamond on a standard 
diamond thermal probe, so the vendor is also marketing a special 
instrument that measures relative transparency in the near-ultraviolet 
range (diamond absorbs and moissanite transmits light in these 
wavelengths).

Moissanite is most convincing as a diamond substitute in small sizes. 
Larger stones still show slight greenish or grayish tones that are 
difficult to eliminate. Careful examination through the crown facets (not 
through the table, since that's oriented on the optic axis) under a loupe 
or microscope should easily reveal facet doubling caused by double 
refraction, as well as very non-diamondlike surface polish and 
inclusions, so there seems to be little excuse for misidentification by a 
trained gemologist.

Best Wishes,