This is good advice. You might consider replacing the mineral crystal with a sapphire one. Sapphire is not as expensive as you might think, although for thick, large, domed crystals it can be. Still worth looking into.
True AR coating is not as effective on a mineral substrate as on a sapphire substrate. This has to do with the index of refraction (IOR) and how the coating is optimized. Genuine sapphire's IOR is anisotropic based on wavelength, and also varies based on purity (quite a bit with our reps). Mineral (i.e. glass) crystals are far outside of sapphire's IOR range. So, when I submit a batch of 40 or so crystals to my coater I ask them to find an IOR midpoint for all the sapphire crystals in the run and optimize the coating based on that midpoint. This is clearly not optimal for the few mineral crystals in the run, but it is really the only method that makes sense for a group of crystals, the majority of which are sapphire. The result on the mineral crystals is less of a color hue and more reflectance off the coated surface than on the sapphire crystals. The optimal way to do it would be to separate sapphire crystals and mineral crystals into different AR runs and optimize the coatings for each. In this case the mineral crystals would perform better, but still not as well as the sapphire.
Too much information, I am sure.