Diamond possesses extraordinary properties, including extreme hardness, thermal conductivity, and mechanical strength. Global industrial diamond production is dominated by synthetic diamond, with important commercial applications in hard coatings and semiconductors. However, the life cycle impacts of synthetic diamond materials are largely unknown. The main aim of this study is to conduct the first detailed life cycle assessments of the typical production routes for nanodiamond and diamond film, which are detonation synthesis and microwave chemical vapor deposition, respectively. The functional units were set to 1 g nanodiamond and 1 cm2 diamond film. A limited number of inputs dominate the assessed impacts: explosives and cooling water for nanodiamond production, and electricity and substrate for diamond film production. Diamond film manufacturers can reduce their global warming, freshwater eutrophication, and terrestrial acidification impacts by 62-71% by sourcing wind or solar instead of global average electricity. However, this comes at the expense of increased mineral resource scarcity impacts at 57-73%. A comparison between nanodiamond and synthetic diamond grit shows that the grit's global warming impact is about 5 times higher, suggesting that nanodiamond is environmentally preferable. The ready-to-use unit-process data from this study can be applied in future studies of products containing these materials.