Prior to the Second Vatican Council altar candles were to be composed primarily or to a significant extent of pure beeswax, with the exact percentage determined by the diocesan bishop. The candle itself was given a mystical meaning: the beeswax symbolized the pure flesh Christ received from his Virgin Mother, the wick symbolized his soul, and the flame his divinity.
However, the current legislation is less specific. The General Instruction of the Roman Missal (GIRM) does not address the composition of altar candles. Conferences of Bishops possess the faculty to determine their make-up, but the USCCB has never employed this faculty to permit materials other than wax in the production of candles, so candles for use in the Mass and other liturgical rites must be made of wax and provide "'a living flame without being smoky or noxious.' To safeguard 'authenticity and the full symbolism of light,' electric lights as a substitute for candles are not permitted" (Built of Living Stones, no. 93). This also applies to the so-called electric vigil lights used for devotional purposes. A bishop would have the authority to make an exception to a living flame in cases of necessity, if, for example, a prison or a hospital had a policy absolutely forbidding open flames.
It should be noted that while an oil lamp may be used to indicate the presence of the Blessed Sacrament in the tabernacle (see GIRM, no. 316), the U.S. bishops have never given permission for the use of oil lamps at the altar. Candles are symbols of the presence of Christ, the light of the world (Jn 8:12) and of Baptism by which we share in his light (Col 1:12), and are also signs of reverence and festivity.