This is a question that often is asked and so I thought I would write a little something for those who are interested or curious about this.
With all candles there is an element of risk as they are a fire hazard. You should never ever leave a candle left unattended and never in the way of a breeze or draft.
Adding crystals to candles does make it a little bit more of a risk as the crystal does heat up within the vessel.
I share information with every purchaser about candle care and how to be diligent around safety..
Crystals in candles can move and they can also fall onto your wick which is an easy fix solution. I use soy wax and so it is softer and easier to manipulate the wick or objects within the jar.
I suggest if you have a large stone within the jar that you take it and place it beside your candle jar as it will more than likely mess with the way the candle burns (causing tunneling or a misshapen burn) and the chances of it heating up is greater. This will not effect the energy as long as the intentions are set and it is placed beside or close to the candle jar..
Also when the crystal pushes the wick to the side of the glass make sure you recenter your wick as burning against the jar can cause it to heat the glass and break.
Smaller crystals within are generally fine and will just sink to the bottom of the candle as the candle burns down.
So yes crystals are safe in candles but they do up the risk especially larger ones. Be mindful to always keep an eye on your candle. Do not let it burn out by itself because having almost no wax in the bottom of the jar, and a crystal will cause excessive heat and may even shatter the jar.