For candles, this is mainly a matter of quality of wax. There are several waxes used for making candles but they are not all of the same quality:
Vegetable wax: extracted from plants (soybean, rapeseed, palm, etc.) is entirely natural, ecological and respectful of the environment. It has no odor, which allows the scent to diffuse without any alteration. However, the wax is not always full white.
Animal wax: only beeswax
Wax from petrochemicals, also called paraffin. Paraffin is the whitest of waxes, it is unnatural and of course, it is the cheapest wax and the most harmful to the environment and the body.
Natural waxes (vegetable and animal) diffuse perfumes better and for longer. Yet another reason to turn to natural candles.
The wicks are also important, ideally made of cotton, they will burn better. Synthetic wicks are less expensive. Sometimes you can find wooden wicks, they crackle a bit, which gives a nice little sound.
Getting more information about candles, I was amazed to see that some of the top brands of upscale interior design or luxury home fragrance still use paraffin waxes. How disappointing is it? So, what justifies their higher price?
Most candles have a label indicating the type of wax used. If there is no detail, trust the color of the candle. If the white is uniform and bright, there is a good chance that the wax is paraffin. Do not hesitate to turn to the wax craftsmen who are responsible for the traditional processes of making candles and their quality.