They (Krsna's cowherd boy and calf expansions) wore armlets on Their arms, the Kaustubha gem around Their necks, which were marked with three lines like a conchshell, and bracelets on Their wrists - SB 10.13.47-48

Expressions researched:
"They wore armlets on Their arms, the Kaustubha gem around Their necks, which were marked with three lines like a conchshell, and bracelets on Their wrists"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

They (Krsna's cowherd boy and calf expansions) wore armlets on Their arms, the Kaustubha gem around Their necks, which were marked with three lines like a conchshell, and bracelets on Their wrists. With bangles on Their ankles, ornaments on Their feet, and sacred belts around Their waists, They all appeared very beautiful.

SB 10.13.47-48, Translation and Purport

All those personalities had four arms, holding conchshell, disc, mace and lotus flower in Their hands. They wore helmets on Their heads, earrings on Their ears and garlands of forest flowers around Their necks. On the upper portion of the right side of Their chests was the emblem of the goddess of fortune. Furthermore, They wore armlets on Their arms, the Kaustubha gem around Their necks, which were marked with three lines like a conchshell, and bracelets on Their wrists. With bangles on Their ankles, ornaments on Their feet, and sacred belts around Their waists, They all appeared very beautiful.

All the Viṣṇu forms had four arms, with conchshell and other articles, but these characteristics are also possessed by those who have attained sārūpya-mukti in Vaikuṇṭha and who consequently have forms exactly like the form of the Lord. However, these Viṣṇu forms appearing before Lord Brahmā also possessed the mark of Śrīvatsa and the Kaustubha gem, which are special characteristics possessed only by the Supreme Lord Himself. This proves that all these boys and calves were in fact directly expansions of Viṣṇu, the Personality of Godhead, not merely His associates of Vaikuṇṭha. Viṣṇu Himself is included within Kṛṣṇa. All the opulences of Viṣṇu are already present in Kṛṣṇa, and consequently for Kṛṣṇa to demonstrate so many Viṣṇu forms was actually not very astonishing.

The Śrīvatsa mark is described by the Vaiṣṇava-toṣaṇī as being a curl of fine yellow hair on the upper portion of the right side of Lord Viṣṇu's chest. This mark is not for ordinary devotees. It is a special mark of Viṣṇu or Kṛṣṇa.