We all know diamonds are the world’s hardest natural substance, but the jewelry industry doesn’t seem to believe it. Judging by the sappy ads popping up on Monday night football, in men’s magazines, and on billboards along Highway 101, diamond peddlers seem convinced there’s nothing more dense and impenetrable than a man’s skull.

“White horse and shining armor sold separately,” the slogans read. “Carve the turkey any way you damn well please” and “She hasn’t kissed you like that since, well, has she ever kissed you like that?”

The ads assure guys that if they surprise their sweetheart with a crystalline rock, she’ll express her gratitude in promising ways: She’ll stop nagging him, start wearing more lingerie, and forgive him for saying that thing about her thighs.

Are women really that easy?

I have a friend whose collie, Mandy, does a great party trick. When asked, “What do girls do for diamonds?,” the dog rolls shamelessly onto her back.

It’s true: Women take a shine to diamonds, and our fondness is as multifaceted as the stones themselves.

“Diamonds are the clearest, purest, most sublime creations on this earth — something raw made into something beautiful,” insisted a friend who fell in love with the gems as a girl, when she was eye-level with her granny’s diamond ring-adorned hands. “It seemed the epitome of ‘grown up,’ and all that is womanly.”

As shallow as it sounds, women adore diamonds for the same reason we swoon over sunsets and wedding cakes and candy-apple red toenails. They’re miraculously pretty — dazzling even.

“They’re mad sparkly,” said one friend.

“I’m distracted by shiny objects,” confessed another.

Still another friend argued that the bling’s not actually the thing. “Women don’t love diamonds merely because of how they look,” she said, pointing out that cubic zirconia is just as flashy. “Diamonds are valued because they are expensive. It’s a status thing.”

I’m not convinced a diamond says “I love you,” but it does say, “Look how much cash my man can hack up for the mere sake of making me twinkle.”

Even if “purchasing power” isn’t your top man-hunting criterion, there’s something about sporting a chunky, platinum-set, emerald-cut diamond that makes even a modern feminist feel like the Princess of Privilege, the Duchess of Don’t You Wish You Were Me.

“Diamonds,” as one of my girlfriends summed it up, “make me feel fancy.”

But that doesn’t mean every giftless Schmo with two months’ salary to burn should be out shopping for ice. Before you shell out for an overpriced carbon nugget, here are some things to consider:

• Diamonds are controversial. And the controversy is über trendy right now. Leonardo DiCaprio’s movie “Blood Diamond,” released this week, illuminates the ways diamond mining in places like Angola and Sierra Leone has been used to finance brutal civil wars or even terrorism. Even Kanye West raps about it: “Little was known of Sierra Leone, and how it connect to the diamonds we own … “
  You can ask your jeweler for a certificate proving his diamonds are “conflict-free” — or you can buy your lady a day at the spa, where there’s no such thing as “unrest.”
• Diamonds are impractical. Call me a romantic, but I don’t want anything that expensive dangling from my body, where it will likely be sucked up in a treadmill, caught on my $15 scarf, or swallowed by my undiscerning toddler.
• Shine is shine, and lots of my friends swore they’d just as soon satisfy their itch for glitz with something that doesn’t need to be insured — sequined flip-flops, say, or a shimmery blush.
  “I would much rather have my backyard fixed up,” admitted one. “Now that would sparkle.”
• Diamonds are a cop-out. Yeah, yeah, they cost a bundle and come in a cute velvet box, but they don’t absolve a guy who’s too lazy to find out what his girl really wants for Christmas.
  Save your money and invest your time in finding out what’s precious to her. It may surprise you.
  “Diamonds do nothing for me,” declared one friend of mine. “I want real estate.”