What does candling mean? Candling is a term held over from the days when people used candles for light. Farmers often held a chicken egg in front of a candle to see if it was fertile. Today an LED light provides a simple way to accomplish the same thing with gecko eggs. The pictures in this article are of various crested gecko eggs in front of a small LED light on a slide-out lighted 30x loupe.
This is an image of an infertile egg. The egg is thinner, oval and not as round as a fertile egg tends to be. It also has a thinner shell with less calcium. When candled all that can be seen is a clear yellow interior.
This is a fertile egg. Note the red round dark area in the lower center of the egg. It has a red dot in the center as well. This is called the cheerio and sometimes just appears to be a single red ring. This egg is mottled on the exterior and the calcification is not even- dark blotches are on the exterior and do not have anything to do with fertility.
This is the other side of the fertile egg above. The dark marks are again on the outside of the shell and you can see the darker area in the bottom center that is thinner than the surrounding shell. Despite the marks and uneven calcification this is still a healthy egg and will likely produce a healthy hatchling. You can't see the cheerio from this angle, so it is important to turn the egg so you see both sides when candling. Just remember which side was up in the nest box so you can incubate it that way.
This is a perfectly calcified white egg with a very healthy cheerio inside. The cheerio is usually slightly left of the 'top' side of the egg if you forget to mark your eggs before you turn them.