A transparent reddish material applied to a clear acetate sheet.
It photographs as opaque so it can be used to block out areas on a flat that should not appear on a particular negative or plate.
It is also used to add cut out areas to a flat that can have an image or halftone negative placed in the cut out to be exposed to the plate.
The cut out area is manually cut into the red transparent material and then the material is peeled off from the acetate sheet, leaving a clear opening in the rubylith.
The cut out area can also be used to expose the shape of the cut out, as a solid or with a screen placed behind it, directly to the plate to manually add an image to the plate.
Used for such purposes as adding screens to the image or a color to a logo.