Single crystal composite synthetic diamond material

A method of forming a diamond composite body, the method comprising:

• providing a first single crystal diamond body, the first single crystal diamond body containing nitrogen and having a uniform strain such that over an area of at least 1×1 mm, at least 90 percent of points display a modulus of strain-induced shift of NV resonance of less than 200 kHz, wherein each point in the area is a resolved region of 50 μm2;
• and
• treating the first single crystal diamond body to convert at least some of the nitrogen to form nitrogen-vacancy, NV−, centres, wherein the first single crystal diamond body has an NV− concentration of at least 0.3 ppm;
• using a CVD process, growing a second single crystal diamond body on a surface of the first single crystal diamond body, the second single crystal diamond body having an NV-concentration less than or equal to 10 times lower than the NV− concentration in the first single crystal diamond body, thereby forming the diamond composite body comprising a first layer of the first single crystal diamond body and a second layer of the second single crystal diamond body.