Some people will wear a black ring on the middle finger on their right hand in order to signify that they are asexual. There’s no particular significance to the color or finger selection. Black was chosen because it was a neutral color, while the right middle finger was chosen largely because a ring on the left middle finger would clash with a wedding band or engagement ring on the left.
Most of the time, the ring is a plain black band of some sort, made out of plastic, stainless steel, or hematite. Some people will opt for a fancier design, with an “Ace” symbol, or with purple and grey highlights.
The black ring symbol began in a thread on AVEN in June of 2005: http://www.asexuality.org/en/index.php?/topic/9175-show-your-pride/