CrustalTrudger t1_j683w0w wrote on January 28, 2023 at 12:09 PM

#1,550,364

Fun idea, but minerals don't work like that. First, some basic mineralogy stuff. Amethyst is just dirty quartz and sapphire is just dirty corundum, i.e., amethyst is a quartz crystal that has impurities (usually iron, but sometimes other metals) and sapphire is a corundum crystal that has impurities (for a blue sapphire, typically iron and titanium). For reference, there are other color sapphires (with different elements subbing into the crystal lattice, producing different colors) and we give other names to corundum with different impurities (e.g., if corundum has chromium in it, it will tend to have a red color, which we call a ruby).

So lets say you take some amethyst (quartz - SiO2 - with some Fe) and sapphire (corundum - Al2O3 - with some Fe and Ti) and put them into a crucible, how hot would you need to get them to melt? Well, quartz ~~(for a rock forming mineral) melts at relatively low temperature of around ~570 C (assuming we're basically doing this at atmospheric pressures)~~ EDIT depending on the type of quartz and the duration of heating, will melt at ~1750 C (e.g., Folstad et al., 2023), but we need to get our mixture up to ~2000 C to melt corundum. Let's say you have the right equipment to do that and you get both your amethyst and corundum into a melt, you've basically made a "melt" consisting of Si, Al, O, Fe, and Ti (assuming that the amethyst was an amethyst because of Fe and not some other metal).

If you start cooling this melt, what's going to happen? Well, you'll start to crystallize things, and effectively you'll crystallize things in the reverse order. I.e., whatever melted ~~first~~ EDIT: last - will start to crystallize first. So in a super simple scenario, as the temperature of our mixture drops below ~2000 C, you might start to get bits of sapphire to crystallize. This is effectively a reflection of one of the basic things we teach in an intro geology class, i.e., Bowen's reaction series, which basically is a progression of minerals you'd expect to crystallize out of a melt containing a mixture of common mineral forming elements (or in reverse, what order you'd expect minerals within a rock to melt as you ramp up the temperature). This progression effectively relates back to the melting/crystallization temperature of different minerals, but also the evolution of a melt, i.e., when a particular mineral crystallizes from a cooling melt because it is thermodynamically favorable to do so, depending on what constituents it "takes up", the composition of the melt will change.

With that in mind, and returning to our specific example, importantly, you've got a melt that has some extra components compared to your original sapphire, namely Si, so chances are you might not even get sapphire (or corundum) back, for example, you might start to instead crystallize an aluminosilicate, i.e., Al2SiO5, specifically probably andalusite since we're doing this experiment at atmospheric pressures) or some other minerals depending on the exact mixtures and conditions as you reduced the temperature. As you continue to cool the melt, finally, you'd probably get quartz, basically using up what ever Si and O were left. Whether this quartz looked anything like amethyst would depend on whether the minerals that crytallized before it had left any iron around. Effectively, what you've done is made an artificial rock, i.e., a mixture of one or more minerals but where the individual minerals are distinct crystals. Also of note, it tends to take relatively specific conditions to grow large crystals that we could consider "gem quality", and chances are, our experiment would not result in this, but instead a relatively fine grained rock with lots of little crystals, so probably not a very pretty rock.

You also might be asking, instead of cooling our melt slowly and letting crystals form, what if we cooled it really quickly, i.e., if we "quenched" our melt? Well, then you've basically formed glass. Chances are it's going to look basically like obsidian, which is a natural form of glass from rapid cooling of melts rich in silicon, oxygen, and aluminum (along with some other bits) kind of like our melt.

Finally, it's worth noting that the material properties for minerals and metals tend to be very different. Those differences in material properties allow metals to be "worked", i.e., you can deform them in a "ductile" manner even at low temperature and pressure to form things like rings. At atmospheric temperatures and pressures, most naturally occurring minerals instead deform "brittlely", i.e., they fracture. So, you would not really be able to form a mineral into something like a band, unless you had a single crystal large enough to just cut a ring shaped object out of this crystal. You can get minerals to deform in a ductile manner, but it takes relatively intense temperature and pressure conditions to do so and not exactly something you can do in your kitchen, unless for some reason you have a diamond anvil cell in your kitchen.

EDIT: For all the people asking me various forms of, "what if you did this other kind of manufacturing technique on minerals/resin/other stuff to get the desired effect?" this is a fundamentally different question than "can you melt two minerals together." The former question is relevant for what OP wants, but is not really for a geologist to answer (i.e., most of us are not professional jewelers, oddly enough). I.e., stop asking me how to make jewellery, I don't know how to make jewellery.

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