A bit of science burbling, if you'll forgive me... You get amethyst instead of clear or smoky quartz if it contains some iron as well as some small amount of radioactive material. (Uranium and thorium are ubiquitous, and in the usual concentrations are relatively harmless. Especially when they're contained inside solid rock, and thus can't really affect their environment.)
Silica, which is the compound that quartz is made of, has the element silicon in its +4 state. Iron is usually +2 or +3. When iron replaces silicon in the crystal lattice, it's still only going to be +2 or +3, and the lattice isn't quite regular at that location. But ionization from radioactivity can take the iron to +4, which fits the lattice better; when the charges from that ionization settle out, the iron may stay in that abnormal +4 state. The extra electron is somewhere nearby, but quartz is a good insulator. And that +4 iron is intensely purple -- the place where the electron "should" be creates a new way for the remaining electrons to interact with light, so yellow light is very strongly absorbed.
If amethyst is strongly heated, that makes the electrons in it more mobile. The electron that the +4 iron had lost is still around the crystal lattice somewhere, and when the amethyst gets hot enough, the iron recaptures an electron, and goes back to +3. The purple colour fades.
![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
no subject
Date: 2019-07-02 05:50 pm (UTC)Silica, which is the compound that quartz is made of, has the element silicon in its +4 state. Iron is usually +2 or +3. When iron replaces silicon in the crystal lattice, it's still only going to be +2 or +3, and the lattice isn't quite regular at that location. But ionization from radioactivity can take the iron to +4, which fits the lattice better; when the charges from that ionization settle out, the iron may stay in that abnormal +4 state. The extra electron is somewhere nearby, but quartz is a good insulator. And that +4 iron is intensely purple -- the place where the electron "should" be creates a new way for the remaining electrons to interact with light, so yellow light is very strongly absorbed.
If amethyst is strongly heated, that makes the electrons in it more mobile. The electron that the +4 iron had lost is still around the crystal lattice somewhere, and when the amethyst gets hot enough, the iron recaptures an electron, and goes back to +3. The purple colour fades.