The Torah requires us not only to ensure that our utensils are kosher , but they must also be spiritually “purified” when ownership is changed from that of a gentile to a Jew. As such any vessels or utensils that are purchased from a Gentile must be immersed (toivelled) in a kosher Mikvah ( unless they were originally made by a Jew).

The Torah in the book of Bamidbar specifically lists various metals in relation to this obligation; therefore the immersion of all metal utensils is a biblical requirement. The sages added to this glass utensils, as they can be made molten like metal, and as such are Rabbinically included in the requirement. Utenisl that can not be made molten such as wood or earthenware are not included in this obligation either biblically or rabbinically. Therefore a blessing is made only on metal or glass utensils. 

The Kashrut Authority takes the view of those decisors who rule that Corelle dishes and Pyrex cookware, as they are also made of glass (albeit toughened), are rabbinically required to be immersed, and a blessing should be made on those utensils.( Some opinions liken Corelle to chinaware and do not require the making of a blessing. KA research however has confirmed that Corelle is indeed a form of glass.)

Porcelain and China crockery and tableware, do not require to be immersed .( This is based upon the rulings of the Pischei Teshuva and Rabbi Moshe Feinstein). Porcelain and china should not be confused with utensils that are glazed with a thick glass coating, which are mentioned in some of the Responsa as requiring immersion , as modern day porcelain and china only have a surface glaze. While the glaze on thickly glazed item could in theory be made molten,and therefore require tevilla as above, it is impossible to make molten the coating on our utensils and as such they have the same status as earthernware that require no tevilla. (Never-the-less in deference to the earlier mentioned view, some customarily do immerse china and porcelain without a blessing.)

Plastic utensils do not require immersion. ( Again plastic can not be melted and re-used like metal or glass. Indeed it burns. However because it gives the appearance of melting some customarily do immerse them.)

Wooden utensils and Earthenware utensils, as mentioned earlier, do not require immersion nor is there any custom to immerse them.