"The contralto singing voice has a vocal range that lies between the F below middle C (F3) to two Fs above middle C (F5) and is the lowest type of female voice. In the lower and upper extremes, some contralto voices can sing from the E below middle C (E3) to two B♭s above middle C (B♭5). Although both men and women may have voices in the contralto vocal range, the word is usually used only in the context of a female singer. The term contralto was developed in relation to classical and operatic voices, where the classification is based not merely on the singer's vocal range but also on the tessitura and timbre of the voice"(Wikipedia)
Ewa Podleś is an example of an operatic contralto:
Within pop and rock music, it's incredibly hard to classify singers. Many of the them don't have formal training, so it's hard to know what they're capable of. Generally, though the deeper, richer voices are altos, and here are three of my favorites:
I was first introduced to the music of Tanita Tikaram through a funky morning radio show that aired on the AM dial, and played a variety of sounds from jazz and world music, to pop, rock and hip hop. Its specialty was those singers that commercial radio had forgotten. (Hint: This was the mid-to-late 90s when we actually still listened to terrestrial radio.) "Twist in My Sobriety" is probably her most well-know song.
I couldn't write a post about contraltos without including the inimitable Grace Jones. Grace Jones is, to me, everything that was right about the 80s (even when I was still too young to "get it").
Joan Armatrading is hands-down one of my favorite vocalists. She's also a rare alto in pop music who has incredible range, but still a rich, earthy quality to her voice.