Do ceramic knives require tevilla?
Earthenware does not need tevilla. Ceramics, such as china, do not need Tevilla. Metals and glass do. But ceramic knives might not fit neatly into any of these categories. As a result, it is not at all clear whether they need tevila at all, and if they do, whether the tevila is midoraysa or miderabanan.
To address this question you have to know basic hilchos tevillas keilim and materials science. The chemistry issue is that ceramic knives are made of Zirconium Dioxide. This material appears in many forms with very different characteristics. It can be made into imitation diamonds, cubic zirconium. It is also photocatalytic. And it can be sintered into knives that are harder than steel. Pure Zirconium is, in the language of science, a metal. As Eli points out, the fact that pure Zirconium is a metal means very little. Pure Silicon is also metallic, but, like Zirconium, does not exist in nature in its pure form. It occurs as Silicon Dioxide- sand- which is definitely not a metal. In halacha, are articles formed from zirconium dioxide metals, and if they are metals, are they included in the metals mentioned in the parsha of tevillas keilim? Or perhaps halacha would categorize them as ceramics, or even glass?
As I said, even if in Halacha it would be viewed as a metal, it is not clear what its status would be for tevilla. The reason is as follows. According to Din Torah, metallic utensils requires Tevilla. But to say that all metal requires tevilla also is not an explicit fact. It is an extrapolation. The Torah only identifies gold, silver, copper, steel, tin or lead as requiring tevilla. What about metals that are not mentioned, such has aluminum or uranium? There are three opinions. Some say they are obligated mi'doraysa and should be toveled with a beracha, some say with a beracha but only Miderabanan (Igros Moshe- Miderabanan. O.C. 3:58, Y.D. 2:164, 3:22, Divrei Chachachim page 189:3:footnote 37, Chai Ha’Levi 4:56:3.) Others say there is no requirement to tovel them at all, like plastic (Divrei Chachachim page 189:3, Emes L’Yaakov Y.D. 120 footnote 51.) For most people, as a matter of practical rabbinics, it has been suggested that one just avoid the issue and be tovel such metals after one has already recited the beracha on metal or glass.
So even if Zirconium were to be defined as a metal, the question would remain. But I don't think that can be assumed. To the uneducated eye, it is no more glass than metal and no more a metal than ceramic. And we cannot look to what chemists or metallurgists or advertising departments say. We need to prove how it would be categorized according to halacha. Since we don't know the rationale for Tevilla, it is difficult to extrapolate.
The bottom line is that it is so different from each of them, that it really ought to be in a class of its own, like plastic, and one could make an argument that it does not require tevilla at all.
It happens that the status of corelle and pyrex, among other pyrocerams, is also not simple. The crystalline structure of these materials resembles ceramic more than glass. In fact, ceramic is subject to differences of opinion regarding kashrus, some considering it to be glass, but many hold that it is Kli Cheres (מ"ב תנא, קסג), which has very different halachos both for tevilla and kashrus. But at least in those cases, the raw material is the same and the appearance and smoothness is the same as regular glass. That is not the case here.
Additionally, I think it is clear that Cheres/earthenware for Tevillas Keilim is not identical with Cheres/earthenware for Kashrus. For Kashrus, the only issue is its absorbtiveness. The Gemara discusses various utensils that are coated with tar, or made from various earths, and uses simple observation of fact to determine whether they have a law of kli cheres for kashrus. For Tevilla, absorbtiveness is not necessarily a determinant. For all we know, Cheres for Tevilla has nothing to do with be'li'ah, but instead with tumah.
More mar'ei makom on porcelain vis a vis kashrus-
Dr. Nachum Stone was kind enough to discuss these matters with his Rav, Harav Rabinovich of Maaleh Adumim. Dr. Stone's presentation of the question is particularly important in that he is a musmach, a talmid chacham, and a dentist who is very familiar with ceramics and metals, and their chemical and physical makeup. This is what he reports.
CORNINGWARE + CORRELLE
ELECTRON MICROSCOPE המראה
כשנשברים, זה דומה לזכוכית לכל דבר.
שאלה: האם סכין קרמי חייב בטבילה
תשובה: החומר ממנו עשויה סכין קרמית, אינו מתכת. אך אין לנו ידיעה ברורה ממה היא עשויה, אם היא עשויה מחומר הדומה לחרס, או חומר הדומה לזכוכית, שאז יש להטבילה, או שמדובר בחומר הדומה לפורצליין, שאין חיוב להטבילה. ובכל אופן, נכון להטבילה בלא ברכה.