Kaustubha — The Divine Gem of Bhagavān's Chest
Kaustubha (Sanskrit: 'found in the waters', from kustu = water) is the supreme divine gem (maṇi) worn eternally on Bhagavān Viṣṇu/Nārāyaṇa's chest. The Bhāgavata Purāṇa and Viṣṇu Purāṇa describe how the Kaustubha emerged from the churning of the Kṣīra Sāgara (the Milk Ocean) during the amṛta-mathana — the cosmic churning in which the gods and demons sought the nectar of immortality. Among all the divine treasures that arose — Lakṣmī, Dhanvantari, the wish-fulfilling cow, the divine physician — the Kaustubha was claimed by Bhagavān Himself and placed upon His chest. The Viṣṇu Purāṇa states: 'kaustubhena ca rājate' — 'He shines with the Kaustubha.' In devotional poetry, whenever the form (divya-maṅgala-vigraha) of Bhagavān is described, the Kaustubha is invariably the first ornament mentioned: 'kaustubha-maṇi vibhūṣaṇa' — 'adorned with the Kaustubha gem.'
In Viśiṣṭādvaita theology, the Kaustubha carries a profound symbolic meaning beyond its dazzling appearance. The Āchārya tradition interprets the Kaustubha as representing all jīvātmans (cit) — the conscious souls who are eternally the property (śeṣa) and ornament of Bhagavān. Just as the gem is inseparably worn upon His chest, the jīvas are inseparably His (ananya-śeṣatva). This interpretation elevates every human soul to the status of a divine ornament on Bhagavān's body — a powerful expression of the śarīra-śarīri bhāva (body-soul relationship between the universe and Bhagavān). The Āzhvārs frequently use this imagery: the soul is Bhagavān's jewel, not a separate, independent entity.
In temple iconography of Śrī Vaiṣṇava divya-deśams, the Kaustubha is a prominent element of the alaṅkāra (sacred adornment) of the deity. It is referenced in the Āzhvārs' maṅgalāśāsanam pāsurams — Tiruppāṇāzhvār's Amalānadipirān in particular describes each ornament of the Lord at Śrī Raṅgam, including the radiance at His chest. Devotees who behold the Kaustubha during alaṅkāra darśan are understood, in sampradāya theology, to be witnessing a symbol of their own eternal relationship with Bhagavān as His cherished jewels.