Method for making diamond layers by CVD

A method of coating a non-refractory and/or non-planar substrate (8) with synthetic diamond material using a microwave plasma chemical vapour deposition (CVD) synthesis technique, the method comprising: forming a composite substrate assembly (1) comprising: a support substrate (2) comprising an upper surface; one or more electrically conductive refractory guards (6) disposed over the upper surface of the support substrate and extending to a height hg above the upper surface of the support substrate; and one or more non-refractory and/or non-planar substrates disposed over the upper surface of the support substrate and extending to a height hs above the upper surface of the support substrate, wherein the height hs is less than the height hg, wherein a difference in height hg−hs lies in a range 0.2 mm to 10 mm; placing the composite substrate assembly within a plasma chamber of a microwave plasma CVD reactor; feeding process gases into the plasma chamber including a carbon containing gas and a hydrogen containing gas; feeding microwaves in the plasma chamber to form a microwave plasma at a location over the composite substrate assembly; and growing synthetic diamond material on the one or more non-refractory and/or non-planar substrates.