What year was the first diamond found?

Diamonds are naturally-occurring, and thus, have been around in recognizable form for billions of years.

Cut diamonds required the human invention of hard tools, likely advanced Stone Age tools. Thus, cut diamonds have probably been around for at least 10,000 years or more.

Note that diamonds, like gemstones and other crystalline forms, do not need to be refined (that is, no chemical action needs to be taken to obtain the "pure" item). They may be cut or polished to enhance their visual (or practical) use, but the raw natural form is easily recognizable and can be obtained in places by as simple a method as picking the raw diamond up off the ground.