[Rockhounds] and the verdict is (was gem silica vs. peruv. opal)

[DonH]

Jeanne Rhodes-Moen wrote:

> both opal and chalcedony fluoresce, but opal is usually pale green or 
> white, while chalcedony is usually blue or white.


Hi Jeanne,

Not to rain on your parade--but opal and chalcedony *can* fluoresce, 
though not always.  And there is plenty of green fluorescing chalcedony, 
usually caused by the "uranyl polyanion".  In fact I don't think I've 
seen any other color than green in chalcedony, but then again I haven't 
seen everything!

Very few minerals always fluoresce; scheelite is one of them, because 
the tungstate (WO4) in CaWO4 is "auto-fluorescing" or "self-activated." 
  In most other cases, the fl. is caused by trace impurities or defects 
in the structure.  Good examples are calcite and fluorite: both of these 
are transparent and colorless in their pure form, but there are many 
elements, especially the rare earth elements, that can substitute in the 
space where the calcite belongs.  These other elements, when they cause 
fluorescence, are called "activators" and "co-activators."  They are 
usually also responsible for the daylight color of these minerals. 
Calcite and fluorite are found in many colors, and fluoresce in 
different colors, and that is the reason why.  In other minerals, like 
feldpsars, you can't tell in daylight that the activators are there, 
since they don't change the daylight color; you can only tell if one is 
fl. and another is not when you use the UV light.  I have collected 
hundreds of feldspars around here in the last 3 years, and only a few 
have fl., and then only weakly.

It must be frustrating to try to assign some names to gems that are not 
well characterized and are mixtures.  On the other hand, they are all 
pretty in your photos, and I'm sure that once you set them in something 
unique, you'll be able to sell them at a good price; you might need to 
get creative with the names, especially considering that some of them 
look like agate from here.

Best of luck,
Don