As a new collector last year I did some looking into the claims of many ebay sellers who claimed their typewriters had glass keys.  Some didn't look like GLASS to me however.  After receiving an old L.C. Smith & Bros. that had several keytops worn through (by long fingernails??) I knew that many keys that might look like glass are, in fact, not glass.

Here are some typewriters that do or do not have glass keys.  Being as how my collecting interests mainly date to pre-1930 typewriters I will pretty much stay with them but feel free to post about key materials on newer and portable machines.

Underwood Nos. 3, 4, 5 all originally had glase keys but, after rebuilding, they might end up with some other keytop material.   Underwood Nos. 6 and later had plastic of some sort.   The early Underwood 3 and 4 bank portables have glass keys too.

Remingtons from the No.2 to the first version of the Model 10 with rectangular tabulator keys have glass.  Subsequent models of the 10 and the 12, 16, 50, etc. have some other material, possibly celluloid.  Rem-Blick and portables 1, 2, 3 have celluloid but the Model 4 seems to have glass (I want one).

Royal used mostly glass keytops but, surprisingly, their first flatbed model had celluloid- then glass on the later Standards (No.1) and later and into the 1940s.  Way to go Royal giving us those nice glass keytops.

L.C. Smith & Bros.  All have celluloid that I have seen.

Woodstock:  celluloid.

Monarch Visible:  celluloid

Harris Visible:  celluloid

Corona 3:  celluloid

This is only a small fraction of old tyepwriters but ones frequently encountered with the metal rimmed keytops.  Many of the early machines have solid composition keytops such as the Smith Premiers, Olivers, New Century, Franklin, Jewett Nos. 4 and 10 and many more.  What material was used on these creamy white and black keytops?  Was it also celluloid?

A glass keytop will be perfectly flat with a reflection from the glass if clean.  A celluloid keytop that closely resembles a glass key will usually have a slightly convex keytop surface without the reflection that a glass keytop will give you nor the look of the letters being UNDER the surface of the keytop.  

A surefire way to tell if glass or another material is to stick a shart pin (sewing needle) into the keytop at a slight angle.  Do this carefully right by the metal rim where any tiny impression will not be noticeable.  If glass the pin will slight off to the side but, if not glass it will dig in and not slide off.  You may be surprised at how many supposedly glass keytops are not actually glass.