When we are talking about Embeds for bath bombs we are talking about the inserted thing that reacts and shoots out colour from the regular bath bomb. Think of it as a mini bath bomb inside a bigger bomb, these are typically made out of the same ingredients as a bomb but a different ratio as their job is to fizz and throw out as much colour as possible. These are usually made ahead of time and since these are typically small and can seem fiddly for just one lot of bath bombs. People usually make large batches ahead of time to be used as needed. Moisture as always should be avoided and putting them in a sealed container gives them the best chance at longevity.
The good thing about embeds is they don't have to be perfect, they can be an odd shape and don't have to be uniform. They are not normally seen and safely hidden inside the bombs.
These can be all different sizes, some people use tab moulds, icecube trays and cut them in half when still wet, people make a whole layered tray and just cut them like a big sand cake (giving you a row of different colours). You could also mix colours in these and get new colours, experiment away! We have even used measuring spoons from a teaspoon to a tablespoon size. You could even try other tools such as a melon baller!