Can mathematical diagrams be copyrighted?

I was planning on making a copy of a geometric diagram that gives a really nice pictorial presentation for the sum of consecutive cubic numbers. It is on page 58 of The Book of Numbers by John Conway and Richard Guy. The book is available on Google Books. Page 58 is accessible but not the diagram. Where the diagram would have been it says copyrighted image. Can they really do this? It would not be all that difficult to give a purely verbal description of the diagram. Would it be legal if I described the diagram on a Web page in sufficient detail that someone could reproduce it?

I was originally planning on putting the picture on a Web page intended for a small group of people in a math meetup. Using geometry software I have been able to create a copy on my own. I would guess that it should be legal to use the picture for a small group for educational purposes. I checked the Web to see if anybody else used the diagram. I found other pictures used to describe the equation, but not this one.