Early lammers were made by engraving the surface of a piece of plastic that was laminated in different color materials. Removing the surface layer allowed the layer below to show through and exhibit the letters or numerals desired. Lammers were made from "laminates". An example is shown below.

The term is common enough that it even appears in some gaming regulation documents. For example, this is from a Nevada document on Gaming Control Standards:

9. All credit issuances are initially evidenced by lammer buttons that are displayed on the table in public view and placed there by supervisory personnel.

Later, chips used to represent cash that wasn't yet on the table were made by other means. For example, by inlaying a design on the chip during manufacture. Tradition though caused them to still be called lammers.