I have a map application that shows locations. You can select what item you want to see data about as well, which edits the icons on the map. In the below picture, you see several locations, and the selected item was a football. The number in an icon indicates how many footballs are at that location, and the colour indicates how many transactions involving footballs have been made at that location. The red locations have a lot of transactions involving footballs, the green ones have very little to no transactions, yellow is in between.
However, I'm using marker clusters as well. The red icon with 100 on the left contains several markers. If you click on the red 100 icon, or zoom in on it, it falls apart into multiple icons, as shown in the second picture:
I have considered adding an transparent outer border to the cluster icons, but while that does differentiate the cluster icon from the marker cluster icon, and outer border doesn't exactly say "there's more markers beneath this icon".
How do I differentiate the cluster icons from the single marker icons, considering they already show so much information?