Jewish law discusses different kinds of fuels and wicks which are invalid for use on the Shabbat, because the flame that they produce is jumpy and unstable. Therefore, there is the concern that perhaps one will try to adjust the flame to stabilize it - and this is forbidden on Shabbat.

All candles which are manufactured today emit a clean, stable flame, and are therefore kosher for Shabbat candles, even if they contain non-kosher (for eating) ingredients.