The chimney crown closes in the top of the chimney. It may be made of concrete, and should be finished in such a way that it causes water to run off the top. The flue liners extend above the crown. You can see a picture on this page.
The crown must be inspected at least annually for cracks and holes. Since its job is to close the top of the chimney, any break in its surface can spell trouble for the homeowner.
Repairs should be made with suitable concrete or commercial sealants available at home improvement stores.
Chimney crowns can also be covered with a stainless steel chase cover. Chase covers are best made from stainless steel and are affixed to the crown using adhesives and hardware. They are an excellent way to protect and preserve the integrity of the top of the chimney. Chase covers must be custom made. Here is a link to a website for a company that makes custom chase covers. Click here.
Stainless Steel Chase Covers are by far the best option. They will not deteriorate under the extreme conditions that chimneys face. They are 100% waterproof, unlike a masonry crown.