There are a number of types of glasses used in stained glass projects.  The two main types of glass are cathedral glass and opalescent glass.

Cathedral glass is a one-color transparent glass.  Cathedral glass can be machine or hand-rolled to a uniform thickness, and can also contain patterns that diffuse light.  It comes in a wide range of colors and surfaces:  granite and ripple textures, for instance, make the piece appear to sparkle in sunlight.  Another technique called “glue chip” is created when cathedral glass has been lightly sandblasted and animal glue applied to the surface.  When the glue dies, it’s pulled off, and flakes of glass will peel off – the result is a frosty appearance on the surface of the glass.  Cathedral glass is great for a “typical” stained glass window or other translucent piece; if you use it in a mosaic, remember the substrate or adhesive will show through the glass.  Many of the windows at Saint John Cantius in Chicago are made of cathedral glass, allowing more natural light to illuminate the interior of the chapel.

Opalescent glass is opaque with one or more colors added to a milk white base.  Go to St. Peter’s Cathedral in Manhattan and you’ll see marvelous examples of how opalescent glass was used to create amazing windows.  Opalescent glass can show shape and shadow as well as color.  It tends to be slightly more expensive than cathedral glass, depending on the number of colors and the pattern created.  Opalescent glass can have two, three, or as many as five colors mixed in a single sheet to create a swirl of color.  The color runs in one direction (typically from top to bottom) because of how it was fed into milling rollers.  The directional color creates an effect that gives the glass the appearance of a “grain.”  (Of course, glass has no grain, so you don’t need to worry about cutting with or against the grain.  The “grain” effect is simply a by-product of the production process.)   This type of glass is usually what is called “stained glass” – it’s what most hobbyists use.

Opalescent glasses come in three basic categories:

Wispy:  more of a clear or cathedral color, instead of solid; it’s very transparent

Translucent:  more opaque than clear; it’s somewhat transparent

Opaque:  dense, mixed color; it can’t be seen through

Because opalescent glass can be seen in reflected light and will also transmit light through it, you’ll need to be careful what colors you choose.  Your piece may look great in reflected light, but when light passes through it, some opaque pieces of glass may simply look black or dark.  In other respects, using opalescent glass or cathedral glass is a matter of personal and visual choice; either glass is easy to work with.

There are a few other types and variations of glass available:

Dichroic glass is a modern glass; it changes color depending on the angle it’s viewed from.  A thin metallic coating makes this glass look silver in reflected light.  It’s difficult to make, and therefore is one of the most expensive glasses you can buy. 

Iridescent glass has a wash applied on the surface of the glass to create a rainbow effect.  (Iridescent glass is also called iridized glass.)  It’s designed to simulate the metallic effect in Favile glass used by Tiffany.  Modern glass manufacturers apply this effect to cathedral, opalescent, and occasionally glue chip glasses.

Dalle de Verre is a form of stained glass, but the glass is poured into molds to create blocks of glass.  Smaller chips can be chopped or faceted into a variety of shapes and patterns.  Dalle de Verre glass is frequently used in jewelry or as accents on boxes, lamps, bottles, and other three-dimensional objects.