He's talking about the refractive indexes. Most stones have more than one refractive index (different values of n for different directions through the material), and hence are dichromic (two-coloured) or pleochromic (many-coloured).

Isotropic pretty much literally means singularly hot, so an isotropic material allows only one wavelength of light through it. Anisotropic means the opposite so anisotropic material lets lots of wavelengths through.

His claims are accurate enough, but he is being deceptive by omission and misdirection.

In truth virtually all gemstones are anisotropic and there is nothing inherently special about the twelve named in revelation. Notice keeps mentioning Diamonds and Garnets as "examples" of isotropic stones, but this is misleading, the truth is that's basically the entire list of known singularly refractive stones. I think there are three in total, diamond and garnet are the best known.

Also his talk about "purity" of light is very strange. Lasers and polarisers enhance/isolate certain properties of light which are useful, however, its a mistake to consider this as making the light more pure, that is just a weird claim. Light has many characteristics but none of them could be said to be more pure than others.

Erich von Daniken would have made a much more fun point out of that.

Dismissing what the old geezer was saying, we might turn to the text itself and ask WTF John was on about. My personal guess is that he is referring to something he has seen, or heard about, that actually existed. People were not stupid in ancient times, even though they lacked the cohesive scientific framework we have today, so they would probably explain technological phenomenons very differently. There are many examples of bizarre objects of a respectable age, such as the Baghdad battery and the Antikythera mechanism. Archimedes of Syracuse was said to have made some sort of "death ray" that he used to set ships in a hostile fleet on fire from afar, but we don't know any technical details. However, we can visualise a concave bronze reflector with some sort of crystal "solenoid" for the makings of a primitive laser. In all likelihood, there's been a bunch of extraordinary talented individuals of the "inventor" persuation all throughout history. They have probably made lots of things that would make even modern people gawk in awe. (And why not? Ignorant people today are ignorant the same way ignorant people have always been - i.e. ready to believe in fairy tales instead of thinking critically about things.) But all such artifacts were unique. There were never any mass production. So we aren't likely to find a lot of them.

If anything, the existence of ancient technological artifacts and "proto-sceintific data indicators" (information that seems to suggest advanced knowledge) should be chalked up to the ingeniousness of the human being - or, at least, those who don't just believe any cock-and-bull story but set out to investigate for themselves instead.

URL to a good resource webpage for dealing with pseudo-science:

www.uwgb.edu/DutchS/PSEUDOSC/Badanal.htm

From my textbook. Physics for Scientists and Engineers, Tipler and Mosca. 6th edition, Volumes 2&3. Ch. 31. Section 3 and 4. 

"A transparent medium is characterized by an index of refraction, n, which is defined as the ratio of the speed of light in a vacuum, c, to the speed in the medium, v [....]"

"The index of refraction of a material has a slight dependence on wavelength. For many materials, n decreases slightly as the wavelength increases [....] The dependence of the index of refraction on wavelength (and therefore on frequency) is called dispersion. When a beam of white light is incident at some angle on the surface of a glass prism, the angle of refraction (which is measured relative to the normal [perpendicular to surface]) for the shorter wavelengths is slightly smaller than the angle of refraction for the longer wavelengths) is therefore bent more toward the normal than that of longer wavelength. The beam of white light is thus spread out or dispersed into its component colors or wavelengths [....]" 

"In an electromagnetic wave, the direction of the electric field is perpendicular to the direction of propagation of the wave. If the electric field remains parallel to a line perpendicular to the direction of propagation, the wave is said to be linearly polarized."

Gee, his arguments are partly based on the premise that these traits are aesthetically appealing. But, clearly, there's nothing intrinsically valuable about polarized light (omg, it's pure!), or different wavelengths (omg, pretty colors!). 

Also, anisotropic means that the material's physical properties are influenced by the direction in which you measure them while isotropic materials are not. Lol, that doesn't sound very pure, does it? God wouldn't have picked impure materials, right?

I sincerely hope you understand that with tits like that, if you play your cards right, you will never have to excuse yourself for anything, ever.