Isinglass is a substance obtained from the swimbladders of fish (especially Beluga sturgeon), used mainly for the clarification of wine and beer. It is a form of collagen. Prior to inexpensive gelatin production and other competitive products, isinglass was used in confectionery and desserts such as Fruit Jelly and blancmange. Isinglass was originally made exclusively from sturgeon until the 1795 invention of a cheap substitute using cod by William Murdoch. This was extensively used in Britain in place of Russian isinglass. The bladders, once removed from the fish and processed, are formed into various shapes for use.
Although very little isinglass remains in the beer which is drunk, many vegetarians consider beers which are processed with these finings to be unsuitable for vegetarian diets (although it is suitable for pescetarians).
(Editor's note: the bottle cap picture comes from the web site of Laura Beamer. Laura is an artist and designer who makes jewelery (including cuff links) from bottle caps. )