Assalaamu alaykum. Is this hadith authentic?
Ibn ‘Abbaas narrated, “We were sitting with Ka‘b one day, and he said, ‘If the hand of one houri were to dangle from the sky, the whiteness and the rings of the hand would illuminate the earth just as the sun gives light to the world. What I said was just regarding her hand, so how beautiful will her white face be when it is adorned with a crown, ruby, pearl, and emerald?’” [Ibn Abi Ad-Dunya]

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

What you mentioned is not a hadeeth which was directly attributed to the Prophet, sallallahu ʻalayhi wa sallam); rather, it is a Mawqoof tradition (a report that stopped at the level of the Companion) narrated by Abu ʻAyyaash – and not by Ibn ʻAbbaas, as you mentioned – on the authority of the Companion ‘Ubay ibn Kaʻb. Al-Albaani classified it as weak in his book Dha‘eef At-Targheeb wa At-Tarheeb. He said:

"[It is Dha‘eef Mawqoof] Abu ʻAyyaash said, 'We were sitting with Ka‘b one day, and he said, ‘If the hand of one houri were to dangle from the sky with its whiteness and rings, it would illuminate the earth just as the sun gives light to the people of this world.’ Then he said, ‘I only said her hand, so how beautiful would the face be; with its whiteness and beauty and loveliness, and its crown with its ruby, pearl and emerald?’” Narrated by Ibn Abi Ad-Dunya, and its chain of narration includes ʻUbaydullah ibn Zahr.

Many scholars of Hadeeth classified ʻUbaydullah as a weak narrator of Hadeeth, such as Ahmad, Yahya ibn Maʻeen, and Ibn Al-Madeeni.

Allah knows best.