One of the most often discussed issues when evaluating a rare coin is the effect that toning has on its grade, and ultimately its value. Unfortunately, for several reasons, there is no easy answer to this question.
When grading a coin, toning comes under the general umbrella of "Eye Appeal." Eye appeal is certainly one of the most subjective aspects of evaluating a coin’s grade. To some collectors, nice original toning is a sign of originality and it serves as an enhancement. Others prefer their coins bright and white, and don’t care for toning at all.
Furthermore, not everyone agrees on what type of toning is attractive. Generally, a dark, blackish or splotchy look is seen as unfavorable, while multicolored, rainbow toning in a crescent or target pattern is usually seen as favorable. Beware however, of toning that looks "too good." Often, artificial toning yields rather bizarre and incredible coloration, which to an inexperienced eye may appear spectacular, but to a more trained eye screams "fake."
Keep in mind that toning is simply the interaction of oxygen and chemicals in the air on the surface of the coin. Natural toning occurs over many decades, while so-called "artificial" toning is simply the same process accelerated by man, either by subjecting the coin to bright sunlight, high humidity or chemicals such as sulphur, bleach or even cigarette or cigar smoke.
Detecting man-made toning can often be tricky, but the PCGS book Coin Grading and Counterfeit Detection notes that the following characteristics are indications of artificial toning:
Assuming the toning is natural, and attractive to most collectors, toning is usually a positive, particularly on older 19th century coins. Many collectors of modern coins, such as Roosevelt Dimes, Franklin Halves, etc... prefer their coins bright, so a deeply toned piece in a mid-20th century series may not appeal to all.
While toning and eye appeal is unquestionably a matter of personal preference, most collectors agree on what is attractive and what is not. For a look at what the graders at PCGS feel is attractive and what isn’t, check out the following link: http://www.pcgs.com/eyeappeal.html