In chapter 1, Ralph and Piggy spot the beautiful conch shell in the lagoon and Ralph enters the water to retrieve it. The conch shell is described as having a "deep cream" color with fading pink spots and a delicate embossed pattern on the outside. The conch shell is also about eighteen inches long and has a slight spiral twist to it. There is also a little hole at the top of the shell with pink lips, which run the length of the shell. Piggy informs Ralph that the conch shell is very expensive and Ralph proceeds to clear the sand out of it before he blows into it, calling the boys to assemble on the platform. The other boys also value the conch shell and elect Ralph to be their chief simply because he is holding it during the assembly. The conch shell becomes a symbol of democracy, order, and civilization throughout the novel and the boys use the shell to take turns speaking at the assemblies.

Well, we get a description of the conch in Chapter 1—it's cream colored, with touches of pink, and about 18 inches from one end to the other. However, if you'd prefer an image, there's a good one here:
http://en.wikipedia.org/wiki/Conch