The most powerful light, pen torch, by QUICKTEST, just like a pen (in fact, the torch pulls out and can be reversed to form an actual pen).
This is the best way of lighting gemstones, especially if you need to shine it precisely into the stone (essential for many optical gem testers) - its finely-focused beam is ideal for this.
It's also good for looking into the inner recesses of antiques or shining through fine porcelain to see if there are any hairline cracks.
Length: 12.5cm long (5 inches)
Diameter at light:12mm
Light / batteries
LED (not replaceable, but then tend to last 30,000 hours)
Takes three LR41 batteries, included.
Most optical gem testers require a light source.
Refractometer (though this is not a monochromatic light)
There are many other applications: seeing the sockets behind computers, visual inspection of PCBs deep in electronic devices, even car engines at night.
This, of course, is not to say that it can't be used as a handy pocket torch for general use, just keep one in your pocket or handbag.