Candle work is magical work.
Candles have been used for centuries in ritual and magic practice. Fire of the candle is a powerful tool, it can ignite things, create and also remove and destroy and you can also use fire to invoke things.
The candle signifies, light, warmth, celebration, love, hope, commemoration.
Is it safe to have crystals and flowers in candles?
In a short answer. YES it is safe. It is safe it they are placed correctly. When I pour my candles I try to add the botanics (and from here on out I will be extra conscious) in already melted wax so that when you light your wick the flame does not ignite the botanics. It is more likely to just melt and drift to the bottom of the jar as the wax melts.