Shiva Lingam's are said to have one of the highest frequency vibration rates of all the stones on earth. As such, the vibration is said to be perfect for purification purposes, whether at home, work or in an area of sacred space.
The shape of the stone represents male energy (knowledge) and the markings, which differ greatly from stone to stone, represent female energy (wisdom). Together the two elements signify the merging of male and female energy and the stones can act as a reminder to us to balance our own male-female energies and create a more harmonious state of being.
In India, the only place where they're found, they're regarded as being sacred and holy stones and are highly thought of in both Hindu and Buddhist belief systems.