When a baby is born how do you know whether the baby is a boy or a girl –
only through one part of the body, everything else looks the same.
Just the genitals tell you ‘This is a boy or this is a girl.
So similarly, to identify something which is infinite they put a stone and that itself will tell you that this is a symbol of Universal energy, the formless.
So, lingam means what? An identification.
Genital is also called lingam because through that you identify, but that is secondary. Simply lingam means an identification,
so they used to keep a stone or a crystal and say this is to identify the inexpressible, that which is infinity.