What does a diamond with a question mark represent?

A diamond (or square) with a question mark in the middle is not an emoticon, but a "replacement character." It is displayed whenever a character is not recognized in a document or webpage.

Most often, a character is not recognized because the character encoding is incorrect or is not supported. Text documents and webpages often define a specific character set, or "charset," which determines how characters are interpreted. For instance, a webpage may define the charset to be "ISO-8859-1." If the text contains characters that are unique to the "UTF-8" character set, they will not be displayed. Additionally, even if a special character like the copyright symbol (©) belongs to the ISO-8859-1 character set, it may not be displayed correctly since the character encoding is incorrect.

Since different character sets use the same codes for standard letters and numbers, most characters will still be displayed correctly even if the wrong character set is defined for a document. Special characters, such as symbols, uncommon punctuation marks, and asian characters are the most likely to be replaced with replacement characters.

NOTE: While the diamond with a question mark is not an emoticon, it is still possible to use the replacement character as part of an emoticon (e.g., ^�^ might be a bird). However, it is best not to use the replacement character in emoticons since people may think it is supposed to represent a different symbol.