Well, not darkness exactly. But I can now block light on command thanks to new "light valves" from Adafruit. These light valves are thin glass sheets that can be darkened on command with a standard Arduino... think LCD screen or welding helmet, except the whole sheet darkens.
Why would I want to darken something, especially jewelry? I've put bright colorful LEDs onto every hat, scarf, shoe and bracelet I could find to make them sparkly. Why would I suddenly want to block light? It took me a while to find a problem for this solution, but I think I've found an interesting use case. What if you are working with objects that already have a beautiful quality of light?
A maker-friend creates lovely fused glass jewelry. The glass does amazing, wonderful things with light. It bends it and scatters it and refracts it. I've put lights behind these pieces of glass to great effect. But some of the pieces, especially the clear or nearly transparent pieces didn't work as well as I had hoped.
Translucent or heavily tinted glass diffuses the LEDs and creates a wonderful glow. But the nearly transparent glass always looked like I had just stuck some LEDs behind it, there was no magic or artistry. They looked cheap and tacky, a sad disservice to the beautiful glass work.