It depends. Per the www.coticule.be<http://www.coticule.be> website:
"The Belgian hones (Coticule and Belgian Blue Whetstone, commonly abbreviated to
BBW) are very closely related. They are mined together. Actually is is
impossible to extract any Coticule without also extracting massive amounts of
BBW. Traditionally, Coticules were always backed by a piece of BBW. Some bonded
together by nature, but most of them glued together by man. The main reason for
this is to reinforce the Coticule part with the stronger Blue stone. Nowadays,
Coticules are backed with a piece of Portuguese Slate, because it is more cost-
effective to use easily available slate tiles for backing a Coticule than to
produce labor intensive backing stones out of BBW rocks. " Snip.
The one I picked out of the $5 bucket at PATINA is clearly naturally bonded, and
the blue side is a decent but coarser stone. But modern stones are apparently
bonded to slate.
From: OldTools on behalf of Ed Minch
Sent: Tuesday, February 13, 2018 2:52 PM
To: Gregory Hahn
Cc: Old Tools
Subject: Re: [OldTools] Coticule is one-sided
Thanks Greg - who would have thought? The maroon/eggplant colored side of mine
sure looks like it’s a sharpening stone.