Seat of the pants gemologist here. I think you could go to a thrift store and find an old flashlight with an old bulb. The dimmer the bulb, the more likely it is warm white. The newer Xenon bulbs that came right before LEDs were probably cooler light. Some of the LED bulbs may be warm white, but my experience is that they are a cooler color temperature. If I were you I would get some color change material…perhaps a synthetic, and try various lights with it until you get the biggest color change from cool to warm light. You can certainly use a warm 2700K CFL as a control for the warmest light. The color temp is noted on the package and probably on the bulb. It would be nice if you could find a warm LED, since they have the most output and efficiency. IDK whether you will find that kind of data for those lights on Amazon or wherever. -royjohn
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