Alchemically, Mercury (Shukra) is considered to be the seminal essence of Shiva. Sulfur represents the generative essence of the Divine Female (Shakti). Alchemy is accomplished by combining the two with other herbs.
A Mercury Shivalinga is considered a sacred object (you get only fakes in the market which just cause mercury poisoning if used) and symbolizes perfect semen retention as Shiva represents the perfect state of stillness where there is no loss of energy of any form. This Parad energy can be valuable for those on the path of sexual energy conservation and transmutation; or even those into sexual alchemy, Tantra, etc.
A practical application of this alchemy is in Kayakalpa - full body rejuvenation. A sect of Aghori Yogis in India use this alchemy to preserve life, body, and youth and extend life for an abnormally long duration, till they attain enlightenment. Their aim is to stay alive and strong, till they complete their goal of enlightenment. In 1997, I visited Benares, India, and met a really old Aghori who had taken a vow of silence for 57 years! He motioned me over and struck my head with his rosary. I experienced a very strong Shaktipat. I sat meditating with him for a week, on and off, and as he was silent, there was no verbal conversation. On my last day with him, he opened his mouth and showed me a silver-red ball that was inserted in his upper palate, which was severed to accommodate the Parad Shivling. He made a hand gesture to tell me he had it there for 30 years! I then realized he was adept at Khechari Mudra, an advanced Yogic practice that few people can master in this day and age.
But then, I have also heard of many monks and commoners dying from mercury poisoning trying to use the mercury Shivlings produced and sold today, as most do not understand the correct process of alchemy which converts the poisonous nature of mercury to make it the immortalizing jing essence of Shiva. So, this field is as close as we can get to the real deal (and not perish from mercury poisoning )