I'm looking for an item to help build calluses (for playing guitar). 
It looks like a coin, a little smaller than the palm of your hand.  It
is metal and has concentric metal ridges on it.  Supposedly you put
your thumb on one side and squeeze your fingertips over the ridges on
the other side.
 
I read somewhere that rock climbing stores might carry something like this.

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Request for Question Clarification by pinkfreud-ga on 15 Sep 2006 10:23 PDT

I haven't found a coin-shaped device like the one you describe.

Would any of these products meet your needs?

http://www.musiciansfriend.com/product/Ruff-Rider-Ruff-Grip-Callus-Builder?sku=423500

http://www.musiciansfriend.com/product/Ruff-Rider-Ruff-Grip-Callus-Builder-Caps?sku=423501

http://www.yesterdaysguitars.com/RuffGrip.html

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Clarification of Question by poocacy2184-ga on 15 Sep 2006 10:39 PDT

Yes, these will work.  Thanks!

I'm glad you liked the callus-building devices that I found. I've
reposted the links below:

Musician's Friend
http://www.musiciansfriend.com/product/Ruff-Rider-Ruff-Grip-Callus-Builder?sku=423500

Musician's Friend
http://www.musiciansfriend.com/product/Ruff-Rider-Ruff-Grip-Callus-Builder-Caps?sku=423501

Yesterday's Guitars
http://www.yesterdaysguitars.com/RuffGrip.html

Here is a good article on building calluses:

Hand and Foot Care: Building and Maintaining Calluses
http://www.handfootcare.com/calluses_building&maintaining.htm

My Google search strategy:

Google Web Search: "callus builder OR toughener"
://www.google.com/search?hl=en&q=%22callus+builder+OR+toughener%22

Google Web Search: "build OR building calluses"
://www.google.com/search?hl=en&lr=&q=%22build+OR+building+calluses%22

I asked a rock-climbing acquaintance about the coin-shaped device that
you mentioned. He said that he thought he remembered having seen such
a product back in the 1980s. The current trend among climbers is
toward maintaining the elasticity of the skin rather than deliberately
toughening the calluses. My friend thinks the device was called a
"Callus Wheel" or "Callus Disc." I haven't found any evidence that
this is still marketed, though.

Best regards,
pinkfreud