She drills holes in them that are about a centimeter in diameter. She bought a special glass drill bit. Some break (maybe 1 in 4ish?) when drilling the holes. She then fills with 50-100 christmas lights. No matter how many she picks, it must be one strand (because the connector thingie for 2 strands is too big). White lights are prettier than the green cord lights but can be pricier and harder to find. The centimeter hole is barely big enough (you can get lights in and out but it's a bit of work). A bigger hole would probably be ideal but it is hard enough to make the centimeter sized hole so it would be even harder and riskier to make a bigger hole.

As for whether you want pictures on one or both sides, it depends if you want to turn it around as you desire or not. On the one hand you get twice the bang for the buck, but on the other you have to remember to alternate it.

Thanks much. Wow....one centimenter is awfully small. Will need to try it. Thanks again.

I wonder what a glass shop would charge to drill these holes? I'd like to make a few, but I hate to mess with the drilling.

where they sell the glass blocks. Maybe they can recommend someone.

I drill glass block and insert either the LED strings of lights or if I paint something on the block, I insert a single 4-watt christmas light.

With the small 6" square glass block, I use strings of 50-60 lights. The large 8" blocks need the larger strings of 80-100 lights. I try to find strings that have some type of blinking bulb type to add to the effect.

For the Christmas light strings, I drill a 5/8" hole. A 1/2" hole will work but it is a little tight. For the larger bulbs, I usually make a 1" hole. After I'm done inserting the lights, I cover the hole with a piece of clear shipping tape and soometimes wrap the edge of the block with wide ribbon - works great!

The link below has a large How-To section with lots of good techniques for drilling glass bottles and glass block. With a good diamond drill bit and the right technques, the drilling is very simple and the lighted block are neat.

Here is a link that might be useful: Diamond Drill Bits for Glass Bottles or Glass Block

I know this is an older thread, but there are glass blocks available that already have the hole in them, and it's a fairly large rectangular hole. They're called KraftyBlocks.

AC Moore stores carry the blocks with holes pre-drilled in them. I made some a few years ago and used 20 - 30 bulb light strands. I used rub-ons for pictres but first sponged acrylic paint all over the block. Then wrapped a wired ribbon around the sides of the block and tied a bow to finish it.