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A niece of mine is an avid amateur Chef (but not yet a knife afi), and this black ceramic knife (a rebranded Kyocera) is one of her favorites in the kitchen, as due to the tougher blade material the edges can be made thinner and thus the knife cuts better than a white ceramic version.
Longtime use however (not always on a suitable cutting board) plus storing it unprotected in a drawer between a bunch of steel bladed knives had blunted the edge to a point next to unusable, complete with quite a lot of (micro) chips and a broken tip.

This is the knife as it was when i received it.
(when you click the pictures 2 x you can see the chips clearly)









This is the knife after sharpening.
I reprofiled the rather bad factory edge to an ever so slight convex edge of ~25 degrees inclusive, and the sharpness is just hairwhittling (only towards the root, not to the point)
It easily slices single layer toiletpaper (torn apart 3-layered version) and a tomato of course.
Removing the chips and setting the new bevel was done with a Tormek T7 fiited with an SB-250 Blackstone (black silicon carbide), refining & convexing with a Paper Wheel coated with 15 micron diamond compound, and semi-polishing with a second Paper Wheel coated with 6 micron diamond compound.










Specs:

Overall length: 11.0 inch (28,0 cm)
Blade length: 5.8 inch (14,8 cm)
Blade thickness: 1,84 mm
Blade type: black ceramic / saber-hollow
Thickness behind the edge: 0,4 mm

The first clip shows the slicing of a piece of standard 3-layered toilet paper, for the second clip i peel off 1 layer and slice the remaining 2 layers, and the third clip shows the slicing of just the remaining single layer of toilet paper:

http://www.youtube.com/watch?v=Ubg_drYHoKE

http://www.youtube.com/watch?v=hUkKfwGFogY

http://www.youtube.com/watch?v=RSkxPA9BK8E

Slicing a tomato:

http://www.youtube.com/watch?v=R3-A0SMcIbw

A small black ceramic backlock folder without any markings (Böker ?) i did in 2013.
This well made knife had practically no edge and no point to speak of when i received it, and even opening mail with it was difficult.
I used the Tormek SB-250 Blackstone graded with diamond to around 1000 grit for freehand reprofiling, followed by 3 Paper Wheels coated with 15, 6.0 and 3.0 micron diamond compounds for refining & polishing.
The new edge fits in the 30 degrees inclusive slot on my Tormek angle gauge, is slightly convex, and can shave the hair on my leg in both directions.









Specs:

Length open: 4.72 inches (12,0 cm)
Length closed: 2.76 inches (7,5 cm)
Blade material: black zirconium oxide
Blade length: 1.57 inches (4,5 cm)
Blade thickness: 2,0 mm
Handle material: Ivory Micarta & Nickel Silver bolsters
Weight: 61 grams

Handle parts & washers fine sanded, polished and waxed, filework & swedge also polished a bit & the blade stropped on hard cardboard with 1.0 micron diamond compound to remove the burr.










Specs:

Overall length: 20,9 cm (8.23 inches)
Blade length after the regrind: 10,0 cm (3.94 inches)
Max blade thickness: 3,1 mm
Brale grind: full convex zero edge
Blade steel: forged 1045 carbon steel
Handle materials: 1 x JB-Weld/Leather/Brass washers, cowbone, Argentinan hardwood, brass pommel
Sheath: brown leather (left-hand model)
Weight (knife only): 98 grams
Weight (knife + sheath): 120 grams

Blade & swedge colored with a blue magic marker before refining step with 1000 grit wet & dry SiC paper, used with oil on glass.






After the refining step:

New JB-Weld "washer" sanded flush, and the blade hand reground to it's original full convex shape on the green silicon carbide side of the Foss 7205 bench stone used with oil.

The first leather washer had rotted and had to be removed.
Then the gap was sanded, cleaned, and filled with some JB-Weld.

Wet ground a new point & a rough new swedge on the side of my Tormek SB-250 Blackstone (black silicon carbide):

This traditional Argentinian Ju-Ca brand fixed blade belongs to a friend of a friend, and as you can see the point was broken off and the handle was also in need of a bit of TLC.

Website of the Argentinian father & son team, who have been making these knives since 1959: http://ju-ca.com.ar/

Here's a link to the review on another forum of that Forever white ceramic knife i did years ago.
In it there are quite a few close-up pictures of the edge, among which several made with a USB-camera.

https://www.talkblade.info/viewtopic.php?f=2&t=24961&sid=db2731afeb2f0f27e9de84ee5ce1c77b

My invention of using fine diamond compounds together with certain types of oil on a naked Paper Wheel has been a game changer in sharpening ceramic knife edges, and a bit later also for steel knife edges.
When i explained my process on the Australian blade forum Vadim Kraichuck was one of the first knife sharpeners who adopted it.

The intrinsic properties & the thickness of the ceramic material most certainly play important parts in useful edge life, but the degree of edge polishing also has proven to be very important yet often overlooked.
Scratches in a ceramic knife apex work as crack initiators, and when those scratches get smaller and smaller due to progressively finer polishing the apex becomes more resistant to chipping as well as keener.
The Forever white ceramic knife in the first two clips has an apex of ~25 degrees inclusive, and it could reverse whittle a chest hair, meaning from hair root to hair point, which is noticeably more difficult than from hair point to hair root.

There are also huge differences in ceramic knives from different manufacturers, and i have found many cheaper ones to be simply not worthy of resharpening.
The two best brands i've encountered so far are made by the Japanese companies Forever and Kyocera (in that order)

According to reports there seems to exist an ever higher quality ceramic knife material, which is made by Swiss company Rahven, but i have not been able to get my hands on one.
Not only does Rahven offer various kitchen knives under their own name, recently they also manufactured a so called "mule" model for Spyderco.

In general ceramic knives are still more chippy when compared to steel knives, but due to the much higher polishing process i have found the resulting edges to be less chippy when compared to factory sharpened ceramic knife edges.
They are also noticeably keener.

With ceramic knives polishing the edges to certain degrees greatly improves their edge retention, and an edge polished up to 1.0 micron diamond compound lasts longer than that same knife polished only to 6.0 micron, all else being equal.

The downside is of course that i need to do 2 more polishing steps (with 3.0 micron and 1.0 micron, all on dedicated Paper Wheels)
Through testing and feedback i have found 6.0 micron to be a good compromise between edge holding and time spent polishing.

For me Paper Wheels used with diamond compounds completely solved the problem of sharpening ceramic knives to a very keen edge.
I only sharpen what i know to be good quality ceramic, and most ceramic edges i polish up to 6,0 micron diamond compound..
For an even keener edge i refine up to 1,0 micron diamond compound.

Some examples i did about 10 years ago:

https://www.youtube.com/watch?v=q4YGGHltloU

https://www.youtube.com/watch?v=dP6fNB_obN4

https://www.youtube.com/watch?v=dP6fNB_obN4

https://www.youtube.com/watch?v=hUkKfwGFogY

https://www.youtube.com/watch?v=RSkxPA9BK8E

Imo sandpaper on a soft substrate will work very well to resharpen an already convexed edge or full convex blade, but to get that convex shape in the first place a hard substrate works much better.

Here the Tormek was used only to grind a new point / slanted edge section on the disassembled blade, using a small knife rest as a guide.

I knew beforehand that the outcome wouldn't be everybody's cup of tea, but for me functionality trumps looks every time.
Grinding the rough convex form with the diamond file (without apexing) took a few hours in total.
Most time was spent shaping & smoothing the blade surfaces on the Foss combination stone and the wet & dry paper on glass.
Visually it's still not 100 % perfect, but the current edge already holds up fine whittling an old piece of hard beechwood cutting board.
A few more pics: