As a collector and player, I use the following criteria to rate dice that enter my hoard. Weight, Coloration & Inclusions, and Overall Look.
Weight: The weight of these dice is pretty decent.
Coloration/Inclusions: Each die will have its own flare given how much of the green inclusion is present. So each die will be unique in its own right yet you'll be able to know they are a part of that set.
Overall Look: These definitely give you the feel of ectoplasm or ooze that's been trapped in the die. Though some die may have more or less of the "ectoplasm" trapped within.
Personal Feels: I like to use these anytime I through an ooze at my players.