al-Baghawi (d.516h) says in Ma'alim al-Tanzil (vol.VI, p.34, Dar Tayyibah edition, 1994):

His saying "... they should not show their zinah (beauty/decoration)" means that they should not show their zinah to other than their mahram. By this what is meant is the hidden zinah, for there are two zinahs: hidden and open. The hidden type is things like the anklets, dye on the feet, bracelets on the wrist, earrings and necklaces. These it is not permissible for her to show, nor for the foreing man to look at. But what is meant by zinah is the place of zinah (in other words, where these jewels are placed).

His saying "... except what is apparent thereof" refers to the open zinah. The people of knowledge have diferred regarding this open zinah which Allaah the exalted made an exception for. Sa'id b. Jubayr, al-Dahhak and al-Auza'i said: it is the face and the hands. Ibn Mas'ud said: it is the clothes, as indicated by Allaah's saying "... take hold of your zinah in every place of worship" (al-A'raf 31), and what was meant by that was the clothing.

al-Hasan said: the face and the clothes. Ibn 'Abbas said: kohl (eyeshadow), and rings and dye (henna) on the hand. That which is from the open zinah is permissible for a man to look at if he does not fear trouble and desire. But if he fears any of that he should lower his eyes. And it has only been made permissible for the woman to display this much for it is not 'aurah (private parts) and she is ordered to uncover this much in prayer. But the rest of her body is 'aurah (private parts) which she must cover. His - exalted and mighty - saying "... and that they beat their khimars" means that they throw their veils "... over their breasts" and their chests so that they may cover their hair, their chests, their necks and their earrings. 'A'isha said: May Allaah bestow His mercy on the first migrating women, who when Allaah the mighty and exalted revealed "and that they beat their khimars over their breasts" tore their garments and used them to cover themselves.

Ibn al-Jauzi (d.597h) said in Zad al-Masir (vol.V, pp.355-356, Dar al-Fikr, 1987):

His saying "they should not show their zinah" means they should not to other than their mahram. Their zinah is of two types: hidden, like the bracelets, earrings, bangles, necklaces and the like. And open, which is what is pointed to in His - the Most High's - saying "except what is apparent thereof". Regarding it there are seven opinions:

The first: that it is the clothing. Abul-Ahwas narrated it from Ibn Mas'ud, and in another wording he said: it is the loose outer garment.

The second: that it is the hands, the rings and the face.

The third: kuhl (eyeshadow) and rings. Sa'id b. Jubayr narrated both from Ibn 'Abbas.

The fourth: the two types of bracelet, which are bracelets and rings, and kuhl. This was said by al-Miswar b. Makhramah.

The fifth: kuhl, rings and die. This was said by Mujahid.

The sixth: rings and bracelets. This was said by al-Hasan.

The seventh: the face and hands. This was said by al-Dahhak.

The Qadi Abu Ya'la said: the first saying is most similar [to what is intended by the verse], and there's textual evidence from Ahmad for it. He said: the open zinah is the clothing, and every part of her is 'aurah (private parts), even the fingernail. This is supported by the prohibition of looking to any part of the foreign women with no excuse, like that he wants to marry her or that he witnesses against her, in which case he looks at her face particularly. But as regards looking at her with no excuse, it is not permissible whether with desire or for some other reason, and regardless of whether that is the face and the hands or any other part of the body. And if it is said: so why is the prayer not invalidated by uncovering the face? The answer is that there is difficulty for her in covering it, so she is excused.

al-Nasafi (d.710h) says in Madarik al-Tanzil (vol.II, p.500, Dar Ibn Kathir, 1998):

"They should not show their zinah" zinah is what the woman uses to decorate herself in jewellery, or kuhl or dye. And the meaning: they should not show the places of zinah, for showing the zinah itself - which is jewellery and the like - is permissible. And what is intended by it is its place, or showing [the jewellery]. And the [jewels] are placed there to show off those places, not for showing of [the jewellery] in itself. And its places are the head, the ears, the neck, the chest, the upper arms, the lower arms, the shin, which are for the diadem, the earrings, the necklaces, the sash worn over the shoulder, the bracelet, the armlet and the anklet. "Except what is apparent thereof" Except what has become customary and is the nature to show, and that is the face, the hands and the feet, for in covering them is clear difficulty, as the woman has no way of getting around doing things with her hands. Also, there is the necessity of uncovering the face, particularly in the issue of witnessing, or trials, or marriage. It is also inevitable when walking the streets, as well as the showing of the feet, and particularly for poor women.

al-Shawkani (d.1250h) says in Fath al-Qadir (vol.IV, p.29, al-Maktabah al-'Asriyyah, 1995): "They should not show their zinah" meaning what they use to adorn themselves, like jewellery and the like. In forbidding the showing of the zinah there is a prohibition of showing where the zinah is worn on their bodies even more so. Then He -may He be glorified- made an exception from this prohibition and said "except what is apparent thereof" The people have disagreed regarding the open zinah and what it is.

Ibn Mas'ud and Sa'id b. Jubayr said: the open zinah is the clothing, and Sa'id b. Jubayr added: the face. 'Ata and al-Awza'i said: the face and the hands. Ibn 'Abbas, Qatadah and al-Miswar b. Makhramah said: the open zinah is the kuhl, the toothpick, the dye till halfway up the shin, and the like, which it is permissible for the woman to show. Ibn 'Atiyyah said: the woman should not show any part of her zinah and hides all of her zinah, and the exception refers to those things which become apparent by the rule of necessity.

It does not remain hidden that the clear meaning of the Qur'anic view is the prohibition of showing the zinah except that which appears of it, such as the jilbab, the khimar and the like -e.g. the jewels and similar things which are on the hand and the foot. If the meaning of zinah was the places [where the zinah is placed], the exception would go back to those things which it is difficult for the woman to cover, like her hands, feet and the like. And that is if the prohibition of showing the zinah necessitated prohibiting the showing of its place by sense of the discourse. However, it is possible that the exception refers to the two places we have mentioned (the face and hands). But if the zinah includes the places [where the zinah is worn] and what the women use to decorate themselves, then the issue is clear, and the exception is for all of it.

(This bit was quite difficult to understand. I think what he's trying to say is that if you take the Qur'an literally, it's prohibiting showing the decorations that women use and nothing more, except that which is seen by necessity, but if you take it to mean the places where the women wear these objects, then the exception is the hands and the face. And if zinah means both the actual decorations and the parts where they are worn, then it means that both are prohibited to show except what appears normally).

He continues, giving the references for the opinions (vol.IV, p.33):

'Abdul Razzaq, al-Firyabi, Sa'id b. Mansur, Ibn Abi Shaybah, 'Abd b. Humayd, Ibn Jarir, Ibn al-Mundhir, Ibn Abi Hatim, al-Tabarani, al-Hakim (who authenticated it) and Ibn Marduwayhi narrate from Ibn Mas'ud regarding His saying "they do not show their zinah" he said: zinah is armlets, bracelets, anklets, earrings and necklaces. "except what appears thereof" he said: the garment and the jilbab. Ibn Abi Shaybah, Ibn Jarir and Ibn al-Mundhir narrate from him that he said: zinah is of two types - the open zinah and the hidden one which only the husband sees. As for the open zinah, it is the garment, and as for the hidden zinah, it is kuhl, the armlets (incomplete...)