Interesting problem…
You will need to make the coin and gem into one Object. There are at least two ways to do this:
Use CTRL+J to combine the gem with the coin. Then you SHIFT+D and move it followed by SHIFT+R several times to make more cons. You’d have a selection problem if you would have used an Array Modifier with this method….
The other way is to take the coin and use a Boolean Modifier to connect the gem to the Coin. Then Apply the Modifier and then make the two materials and delete the original gem. Now use the Array Modifier method.
