Double diamond glass is 3 mm or 1/8 inch thick and is most often used in cabinets such as those found in bathrooms and kitchens, and in standard windows. Single diamond glass is 2 mm or 1/6 inch thick. Single diamond's primary use is in picture frames; in the latter thickness, you can choose "non-glare" glass, which dispels reflections.
According to White Rock Glass, double diamond and single diamond glass both come in clear, gray, patterned, bronze or mirrored versions.
When you hit double diamond glass, it will shatter. For additional safely, you can obtain most flat glass--double diamond included--in tempered and laminated versions. When tempered glass is struck, it fragments but does not shatter. Laminated glass takes two sheets of glass joined by a central resin layer; if the pane is broken, the pieces stay together.
If you plan to use mirrored glass, you may wish to choose a thicker glass, as much as 1/4 inch. The stronger glass helps reduce distortion in the reflection. The thicker the glass, the more you can do with it, including using it in shower doors and tabletops.