CC Madhya 8.294

tāmā, kāṅsā, rūpā, sonā, ratna-cintāmaṇi
keha yadi kāhāṅ potā pāya eka-khāni
Synonyms: 
tāmā — copper; kāṅsā — bell metal; rūpā — silver; sonā — gold; ratna-cintāmaṇi — touchstone, the basis of all metals; keha — somebody; yadi — if; kāhāṅ — somewhere; potā — buried; pāya — finds; eka-khāni — in one place.
Translation: 
Actually, these conversations are like a great mine where, from a single place, one can extract all kinds of metals — copper, bell metal, silver and gold — and also touchstone, the basis of all metals.
Purport: 

Śrīla Bhaktivinoda Ṭhākura gives the following summary of the conversations between Rāmānanda Rāya and Śrī Caitanya Mahāprabhu. Rāmānanda Rāya replied to five questions of Śrī Caitanya Mahāprabhu, and these questions and their replies are recorded in verses 57-67. The first answer is compared to copper, the second to a better metal, bell metal, the third to a still better metal, silver, and the fourth to the best metal of all, gold. But the fifth answer is compared to the most valuable gem, touchstone, because it deals with unalloyed devotion, the ultimate goal of devotional life, and illuminates the preceding four subordinate answers.

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura points out that in Vrajabhūmi there is the Yamunā River with its sandy banks. There are kadamba trees, cows, Kṛṣṇa’s sticks with which He herds cows, and Kṛṣṇa’s flute. All of these belong to śānta-rasa, the mellow of neutrality in devotional service. There are also the direct servants of Kṛṣṇa, such as Citraka, Patraka and Raktaka, and these are the embodiments of service in the mellow of servitude. There are also friends like Śrīdāmā and Sudāmā, who embody service in fraternity. Nanda Mahārāja and mother Yaśodā are the embodiments of parental love. Above all of these are Śrīmatī Rādhārāṇī and Her assistants, the gopīs Lalitā, Viśākhā and others, who embody conjugal love. In this way all five mellows — śānta, dāsya, sakhya, vātsalya and mādhurya — exist eternally in Vrajabhūmi. They are also compared, respectively, to copper, bell metal, silver, gold and touchstone, the basis of all metals. Śrīla Kavirāja Gosvāmī therefore refers to a mine eternally existing in Vṛndāvana, Vrajabhūmi.