The surface roughness depends on the material and processing method.
For example, when machining with a surface grinder, alumina is about Ra1.6 and silicon carbide is about Ra0.4.
Fine ceramics are basically machined using diamond tools, but they vary depending on the state of the tool and the machining conditions, making it difficult to control the surface roughness.
If you want to roughen the surface, there are some methods such as changing the machining tools and conditions or blasting.
If you want to polish the surface, you can use buffing or lapping.