How To Make Ruby in a Microwave at https://www.youtube.com/watch?v=ybcdRQmQcHQ
It looks like the aluminum wire is burning (oxidizing) to aluminum oxide, and melting some of the close mixture. If you use coarse grained aluminum powder, and mix with the oxides, it might work longer and more uniformly. The melting point of aluminum oxide is 2072 C and of chromium oxide is 2435 C. The heat of formation of aluminum oxide is 1675.7 KiloJoules per mole, and a mole of Al2O3 is 101.96 grams. Burning aluminum in air is exothermic, so you get concentrated heat from that. The microwave might be 1000 Watt. The “latent heat of fusion” of Al2O3 is between 620 and 1360 KiloJoules/KiloGram or about 1000 Joules/gram or about 1000 WattSeconds per gram. If all the energy of the microwave went into melting and a bit from burning aluminum in air, one might expect a few grams of material. Weight your reactants. Weigh your products. With a reference UV source you can use a camera to count the intensity and number of pixels as a proxy for the amount of material.
The fluorescence of ruby is a doublet at 692.8 and 694.3 nanometers (nm)
Here is an American Chemical Society paper (Sept 2013) describing a student experiment to make ruby. Microwave Synthesis of a Fluorescent Ruby Powder at https://pubs.acs.org/doi/abs/10.1021/ed3008389
I wish I could be more helpful. You might try an induction furnace. You might try metal mirrors and cavities to focus the microwaves. You might pull the microwave out and have a separate furnace. You could try lasers to melt a moving front and make wires of ruby. Tungsten melts at 3422 C. But your method of focusing the microwave energy inside the powder is good. I was afraid you were going to burn your fingers.
You gave lots of ideas and encouragement. Thank you.
Please be careful, finely divided aluminum might be dangerous in a microwave. There are several references that mention “finely divided aluminum” “microwave” including some patents. There are MANY references to “aluminum powder” “microwave”. And “microwave” “sintering” is a rich field of research, many aimed at manufacturing fine ceramics. A plasma and a flame are different aspects of rapid reactions.
Richard Collins, Director, The Internet Foundation