Pumice, perhaps light, which forms moving blobs (4,4)
I believe the answer is:
lava lamp
'light which forms moving blobs' is the definition.
'lava lamp' can be an answer for 'light' (I've seen this before). I am not sure about the rest of the definition.
'pumice perhaps' is the wordplay.
'pumice' becomes 'lava' (I've seen this in another clue).
'perhaps' becomes 'lamp' (I am not sure about this - if you are sure you should believe this answer much more).
'lava'+'lamp'='LAVA LAMP'
Can you help me to learn more?
(Other definitions for lava lamp that I've seen before include "decorative novelty item" , "Light appliance, popular in the 1960s" , "70s novelty domestic light" .)