From our text Nectar in a Sieve: "she...placed in my hand a small stone lingam, symbol of fertility." Why did Rukmani's mother give her a stone lingam?

Rukmani is believed to no longer be fertile and the lingam objectifies the ability to have children.
The lingam is a medical herb that will help Rukmani's morning sickness. Rukmani's mother loves lingam and finds it to be very beautiful.