One of the most present effects in fireworks, are crossettes. A crossette star or comet basically consists in splitting a star in 4 similar parts by the explosion of a small bursting charge ( usually flash powder ).
The crossette stars or comets are made using the desired star effect composition in the pressed form and they contain a cavity with a cross format. That cavity is filled with some burst composition like flash powder. The whole star / comet body is protected with kraft paper and the bottom ( the cavity side ) with a cardboard disc, so, only one side is exposed to initial fire and usually it is primed for better ignition start.
Crossettes are used both in Shells and also in Roman Candles (look this post to read more about Roman Candles) and essentially the efect is a burning star with an explosion at the end splitting the remaining of the star in 4 similar parts.
Below, you can see a draw explaining the structure of a crossette:
Below, you can see what the effect looks like in a shell:
What a beautiful effect!
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