It's entirely ceramic, just like aluminium oxide, except it is more biocompatible as far as I'm aware.

A biologic dentist here in my country uses ceramic tooth implants "Zirconia" (which is zirconium oxide).

He has studied extensively not only about amalgam/mercury, but also alternative dental materials, of which certain ones can also be problematic in patients. So he tests patients for biocompatibility before implanting any materials.

This is what is said about this material in regards to dental implants on his website:

http://www.dr-jacques-imbeau.com/metalfreeimplantology.html

Quote
Although titanium is commonly used for dental implants, it is a metal and is thus susceptible to corrosion in the mouth. Ceramic materials such as zirconia offer many advantages over titanium.

Why consider zirconia ceramic dental implants ?

- Cosmetic superiority
- Excellent osseointegration
- Bio-inert *
- Simple, single piece installation
- Improved patient recovery time
- Comparable costs (titanium vs. ceramic implants)
- Certified and approved for use in Europe and New Zealand
- As strong as titanium without the disadvantages

So why have a metallic implant that is the wrong colour, can corrode, conduct electrical currents and react with other metals when you can have a white, bio-inert*, high performance zirconia ceramic implant ?

High purity zirconia demonstrate excellent general biocompatibility, but like any other dental material designed for human implantation, it should be tested for individual biocompatibility prior to placement. Individual biocompatibiliy may vary from general biocompatibility depending on genetic characteristics, health status and other individual biological variables.


Here's some more info regarding this material.
http://www.dentist-planet.com/zirconia-implants.php