grain (e.g. rice)
顆 centers on a small, countable grain or particle, like a single grain of rice. It is used as a counter for small roundish objects.
顆 combines 果 (fruit, result) with 頁 (head), originally referring to small, head-like round objects such as grains. The exact historical development is uncertain, but the phono-semantic structure with 果 contributing the sound カ is likely.
Picture a fruit (果) that is actually a head (頁) made of tiny grains. Each grain is a single 顆.
For カ, imagine a car (カ) filled with grains of rice, each grain counted as one 顆.
granule
condyle
stamp; seal
agranulocytosis
spherulite
Condylarthra; extinct order of mammals
granules (medicine)
granular layer; stratum granulosum
granular
granular
granulocyte
agranulocytosis
granulocyte
granule cell