Marilyn Monroe made that phrase famous. Diamonds are a girl"s best friend. That still makes a lot of sense. Here"s why.

Reason #1 diamonds are a girls best friend because they are so beautiful. Fiery, romantic diamonds light up her finger. White diamonds look beautiful with outfits in every color.
Reason #2 diamonds are forever. There are lots of beautiful things around. Why diamonds? Roses are beautiful, but they fade. Dresses are gorgeous, but after childbirth, or as she ages, that beautiful dress would not look as beautiful on her anymore.
Reason #3 diamonds are precious. Sure, moissanite may be what a girl might buy for herself, because of its lower price, but moissanite is man made. Diamonds are rare, natural stones. The diamonds that girls love are of the highest quality. The lower grade diamonds may be used in tools, but women look at the quality of the diamond. See http://www.buy-diamonds.net/choosing_a_quality_diamond.htm for details on choosing a diamond. A large, high quality diamond would definitely impress.
Reason #4 diamonds are romantic. Especially when he is proposing to her with a diamond worth two months of his salary. It means sacrifice for love on his part. The fact that he loves her enough to save that sum to buy that diamond for her is a very romantic gesture that she will remember for years to come, each time she sees that diamond on her finger.
Reason #5 diamonds say that her man can provide for her very well. From the days when men would go out and hunt for animals to feed the family, to today, when most men go out there to bring in the money to support the family, women love being cared for. Being provided for. In many cultures, women look to marry a rich man. A man who can provide for them. If a guy can afford to give her diamonds, he can easily provide for her daily needs and for that of her children.
Reason #6 diamonds are practical. Like the song, diamond"s are a girl"s best friend. A kiss will not pay the bills. A rose isn"t going to pay for her rental. A diamond, on the other hand, in times of emergency, can be a life saver. A single diamond can be sold for enough money to pay for a woman"s bills when times get tough.
Reason #7 diamonds are a girl"s insurance. Ok, so now, there are insurance policies in paper, that promise to pay for her old age medical bills etc, but diamonds are a lot more beautiful. She holds and sees them. She wears them. Everyone admires them. They assure her that even when she gets old, when she loses her looks, even if her guy leaves her for a younger woman, the diamonds would still be with her. A collection of diamond jewelry assures her that in her old age, or she is in dire need for cash, a piece of diamond jewelry could be sold, or pawned for much needed cash. Diamonds are a girl"s insurance.