Without any background/context, how does one decipher the meaning behind the gift?
Is the guy a bf, a close friend, someone who is courting you, someone u've just met etc etc?
Without knowing the background, you can very well infer that
a) that the guy probably has romantic feelings for you (in this case, he shd probably be yr bf or someone who is pursuing u) and has been brainwashed by the ads on tv that diamonds are the best way to show his affections..
b) he wants to get into yr pants (shower you with expensive gifts and hoping for reciprocation)
c)he is just a close/good friend who is aware of your taste for nice jewellery
d) none of the above because i cant think of any other reasons except maybe his family owns the jewelery shop and he wants to get rid of stock.