A zodiac candle is a candle for a birth sign, usually with a gemstone set into or supplied with the wax. They make good gifts because a birth sign is the one piece of esoteric information almost everyone knows about themselves.
The stone-to-sign lists are not old
It is worth being straight about this, because most shops are not. There is no surviving ancient list assigning crystals to zodiac signs. The pairings you see today were assembled in the twentieth century, and different authors assign different stones to the same sign — one book gives Leo tiger's eye, the next gives it carnelian, a third gives it citrine.
We have written about that at more length in there is no ancient list of zodiac crystals. The short version is that the tradition is real as a modern practice and false as an ancient one, and a shop that tells you otherwise is guessing or embellishing.
Which is not a reason not to buy one
A zodiac candle is a nice object with a stone in it, given to someone whose birthday it is. That is a perfectly good thing for it to be. The recency of the correspondence only matters if somebody is claiming otherwise — and for a gift, the sign is doing the work of making it feel chosen rather than generic, which it does regardless of when the list was written.
Choosing one as a present
Go by the sun sign, which is the one people know from their birth date. If you want to be more specific you need a birth time and a chart, and at that point you are buying something more involved than a candle — our birth chart tool will work it out if you have the details.
Practically: check whether the stone sits in the wax or comes separately. One is a candle you burn down to reach the stone, the other is a candle and a stone in a box, and people have opinions about which they wanted.