"Just as among Goddesses, Uma (Shakti of Shiva) is considered to be the highest, among all Shivalingas, one made of clay is said to be the best."

* Somnath Jyotirling ~ Gujarat *

यथा देवीषु सर्वासु शैवीशक्तिः परा स्मृता ॥ 
तथा सर्वेषु लिंगेषु पार्थिवं श्रेष्ठमुच्यते ॥ 

Just as among Goddesses, Uma (Shakti of Shiva) is considered to be the highest, among all Shivalingas, one made of clay is said to be the best.