Granite paper

Granite paper: A paper with small colored fibers added when the paper is made. This paper is used as a deterrent against forgery.

Granite paper -- Not made of stone, but paper with colored fibers running through it.

Granite Paper - Paper containing small colored fibers that is used to deter forgery.
Grade - A judgment of a stamp's level or range of imperfections.
Gravure - See "Photogravure." ...

Granite Paper
Granite paper is a form of paper that contains numerous colored fibers. In most cases, granite paper has been produced as a form of security device to deter postal counterfeits.
Graphite Lines ...

Granite paper is rather unique. It looks like wove paper but has blue and red silk fibers throughout.
Perlure paper is very thin, not much thicker than an onion skin. It is usually hard and brittle.

Granite Paper of Switzerland SC#65 magnified 35x
Granite paper has colored (typically red and/or blue) silk fibers added to the furnish.

Papel con filametos: (Sp.) granite paper.
Papel Delmeina: (Sp.) paper, invented by a Belgian engineer, made in Spain and used for Civil War postal tax and charity issues; ...

See also: Granite, Used, Stamp, Printing, Watermark