According to Shiva Purana – Two forms I have, Shiva is My Physical manifestation, and the lingam of luminosity represents My unmanifest Abstract reality. I can also manifest in propertyless, formless, infinite, and unprecedented forms. I manifested as Lingam to reveal my universality. Shiva manifestation shows My Supreme divinity. Sphatik Lingam is My symbolic form and it gains one access to me.
One who installs Red Jasper lingam with Green Jade base with true devotion gets unified with my divine reality and achieves final salvation in the process. The idol of Shiva form for Lingam worship is for those Faithfull who can not comprehend my abstract reality.