It looks like perlite. You can buy a powdered perlite from pool supply stores that is used as a filter aid. I have used it for rooting cuttings but not as soil for growing any plants. You could try 50/50 silica sand/powdered perlite. If you completely enclose it you would only have to water once every 6 months or so with cactus.

I dont know a great deal about terrariums but I know a little about cacti. Cacti are arid plants, where they live there is very little humidity. Although most varieties you find at your big box stores are fairly adaptable. With that being said, dont get discouraged if a few die from rot. Again I dont know much about the needs of terrariums soils but cacti need loose soil. Perlite and sand are great for drainage as long as they are the right size grainules. You want to avoid anything smaller then an 1/8 inch and no larger then 1/4 inch. Silica sand and crushed perlite will compact very tightly and hender drainage and root growth. I use a mixture of perlite, peat and turkey grit. Turkey grit can be found anywhere animal feed is available.I generally make a mix of equal parts. Remember, a cactus is more likely to be killed by the abundance of water then the lack of it.

In the picture, it appears that they used white aquarium gravel and a fine white sand, probably silica. These are tops dressings to make the terrarium look snazzier not actual components of the soil mix.

I hope this is useful to you in some way or another. Keep us posted on your progress with pics!

plant_junkie

Also another idea is Turface or even CAT LITTER!! I know it sounds wierd but certain brands of cat litter is a hi temp baked clay. You ca use it as "soil" bc it hold moisture really well and supplies tons of oxygen. I use it as a soil for my bonsai, terrariums, and even in my outdoor garden to break up dense soil. I happen to like the color of it but sometimes ill cover it with a top dressing. The best I have found is an automotive oil absorber (same as cat litter) called oil dry. I bought it at Autozone or NAPA and it was 3$ a bag!!! oh and my succulents and cacti love it!!