Okay, so this is really two questions, one having to do with electrical engineering, and the other having to do with materials processing (a name I just made up). I was at a museum, and I saw on display a rotating platform that held a slab of clear glass. Inside the glass there was a milky 3D image of a steam locomotive. The locomotive pattern had the look of etched glass, but heretofore I have only seen etching on the surface of glass, and this pattern was most definitely in the interior of the block of glass. There were no seams in the glass. As the glass rotated, the locomotive lit up in four colors: red, yellow, green, and blue. A museum worker lifted the glass up and let me look at the clear platform that it sat on. Under the rotating platform there were four fixed LEDs, shining in the aforesaid four colors. So the EE question is: when and how did they make blue LEDs? I can remember red and green being common when I was young, and then they came out with yellow (a.k.a. amber), which if I recall somehow combines the red and green technology to put out a mixture of light that looks yellow. But blue LEDs are something new to me. Second question: how do you etch (if that is even the right term for it) the interior of a chunk of glass?