The exact compostition of a candle flame is actually very difficult to work out - flames are a dynamic mixture of reacting gasses and get their colour from microscopically small particles of carbon. The problem is that these carbon particles are actively reacting in the flame, and by the time they have left the candle they have reacted to form CO2 or stuck together to form soot. So what exactly is going on in the flame has remained quite obscure.
This point that we don't really know what is going on in a candle flame was made to Wuzong Zhou, Professor of Chemistry at St Andrews University, he took this as a challenge and set about finding out.
The big problem was extracting the particles from the centre of the flame quick enough and stopping the reaction. Zhou developed a new sampling technique which involved using anodic aluminium oxide films - these are a piece of aluminium which has been oxidised electrochemically to form a honeycomb like structure, which contains many holes of controllable size, generally a few 10s of nanometers across. The back of this film can be removed with phosphoric acid leaving, essentially, a filter. He built a sandwich using two layers of filter - the first with 80nm and the second 40nm holes - and put it into a candle flame for about 1 second.
The films were then studied using an electron microscope to identify the products found in a flame. Zhou found tiny pieces of graphite, which you would expect in soot as this is the most stable form of carbon at atmospheric pressure. He also found fullerenes - ball shaped cages of carbon known as "bucky balls". The big surprise was finding tiny diamonds. However, as they are only a few nm across, just a few tens of atoms in diameter, these diamonds are probably not very useful for that engagement ring.
Understanding how these diamonds are nucleating in a flame may be important in understanding the growth of diamonds in the process of chemical vapour deposition, this is used for industrial purposes ranging from scalpel blades to heat sinks and high temperature electronics, but could also be used for jewellery - bringing that engagement ring that little bit closer.