Designer's Guide Results
The following is a list of materials, selected and ranked per your specification.

MaterialDielectric Strength
Silicon Nitride17700 ac V/mm
Fused Quartz / Silica1270 ac V/mil
Sapphire1200 ac V/mil
Boron Nitride950 ac V/mil
Macor785 ac V/mil
Aluminum Nitride17 kV/mm
Steatite230 - 447 ac V/mil
Zirconia-PSZ250 ac V/mil
Alumina 99.9%230 ac V/mil
Alumina 94%212 - 220 ac V/mil
Reference Materials
Diamondn/a
Stainless Steeln/a
Tool Steeln/a

All properties values are at room temperature unless otherwise noted. Engineering data are representative, and are not intended as absolute nor warrantable. Data shown is blended from multiple sources and therefore illustrates the marketplace. The relative rankings are approximate and are based on representative values if the particular material has a range of values for a given property.

Wikipedia Definition(s) :  Dielectric Strength;

Check Different Properties

Please Note:  This Designers guide is compiled of materials currently machined by Insaco and a few others included for reference.