Parece que en este caso en concreto, las empresas yeseras en cuestión se dedican a fabricar "placas de yeso" (o pladur, como también se conoce en España), no a la extracción y fabricación del yeso en sí. Hay muchas referencias de la noticia en Google.

http://www.consumer.es/web/es/economia_domestica/2002/11/28/...
La UE multa con 478 millones de euros a cuatro empresas yeseras por pactar precios

Se trata de la segunda sanción más elevada impuesta por el Ejecutivo comunitario
(...)
Las empresas sancionadas son la francesa Lafarge, multada con 249,6 millones de euros; la británica BPB, 138,6 millones; la alemana Knauf, 85,8 millones; y la belga Gyproc Benelux, 4,3 millones.
(...)
***Las placas de yeso*** son un producto manufacturado que se utiliza como material de construcción prefabricado o en el bricolaje doméstico.

http://europa.eu/rapid/pressReleasesAction.do?reference=IP/0...

Commission imposes heavy fines on four companies involved in ***plasterboard cartel ***

The European Commission today imposed fines totalling € 478 million on four companies which operated a long-running cartel on the market for ***plasterboard***, a product which is widely used in the building industry and by DIY practitioners.
(...)
Following a detailed investigation during which it carried out surprise inspections in 1998, the Commission concluded that, between 1992 and 1998, BPB PLC (United Kingdom), Gebrüder Knauf Westdeutsche Gipswerke KG (Germany) and Société Lafarge SA (France) participated in a plasterboard cartel in the United Kingdom, Germany, France and the Benelux countries. Gyproc Benelux SA/NV (Belgium) joined the cartel in 1996.



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Note added at 24 mins (2007-09-12 09:58:06 GMT)
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En fin, si se quiere ser fiel al texto original, gypsum manufacturer.

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Note added at 27 mins (2007-09-12 10:01:07 GMT)
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O "gypsum products manufacturers".

http://www.advfn.com/news_BPB-Lafarge-Knauf-face-EU-fines-ov...

Bpb Industries BPB, Lafarge, Knauf face EU fines over plasterboard cartel - report

LONDON (AFX) - BPB PLC, the building materials group, is facing a fine from European regulators for taking part in a conspiracy to fix the price of products used in the construction of walls and ceilings, the Financial Times reported citing people close to the action.
Lafarge SA, the French building materials group, is also poised to receive a heavy fine from European regulators.
Germany's Knauf could also receive penalties for participating in the cartel covering the market for plasterboard and other gypsum-based materials, the article added.
Other companies could also be facing fines from Brussels. The European Commission declined to comment yesterday but people close to the case said that Lafarge could be fined