I'm having trouble finding the available focusing screens for the Canon 6D.
Are the canon 6D focusing screen compatible with the 5D Mk II focusing screens? Is there a microprism focusing screen for the 6D?
According to Canon USA's online support page (see the note below), the only focusing screen compatible with the Canon EOS 6D is the one supplied with it, the Eg-A II. Also according to Canon USA's online support page, the focusing screens available for the Canon EOS 5D II are only compatible with the 5D II.
However, according to page 312 of the EOS 6D Instruction Manual, the Eg-D (Precision Matte w/Grid) and Eg-S (Super Precision Matte) are compatible. They are also listed as available accesories on page 354. If you install one of these focusing screens, be sure to set the C.Fn.3- II to match the correct focusing screen so that the exposure metering system can compensate for the different amount of light the screen allows to pass to the light meter in the pentaprism.
The last micro-prism screens I'm aware of that Canon made were the Ec-A (matte w/out split image) which is listed as compatible with all 1-series bodies including the 1D IV and 1D X and the Ec-B (matte w/split image) which fit 1-series bodies through the 1D III and the 1Ds III, but not the 1D IV.
The newer bodies from the 7D onward use a transmissive LCD screen to project most, if not all, of the viewfinder information onto the focusing screen. Before the 7D the markings were etched onto the focusing screen and LED lights were used to illuminate active focus points, etc.
http://usa.canon.com/cusa/sna/consumer (click 'Camera Accesories--> Focusing Screens' and then scroll through the list.) In general the Eg series are for the 5DII, the Ec series are for the 1-series bodies, and the others are for even older cameras. Notice that there are no replacement screens listed for the 7D and 5DIII on Canon's official web site because the focusing screens on these models are not considered user removable, and Canon does not provide Custom function settings to compensate for exposure metering when using third party focusing screens.