Rosemary essential oil is one of the basics. I've probably got an ounce or two within a dozen feet of me. It's got a nice smell.

16oz of pure rosemary essential oil would last someone a very long time. It wouldn't be marketed in a bottle like that. On the other hand, rosemary-infused olive oil is very much a thing for cooking. I've made it myself and given it out at Christmas. Great for putting on bread with herbs and balsamic or you can cook chicken or pork in it.

So while I agree with Trombone that there's all sorts of health effects rosemary oil is accused of, the size of the bottle leads me to believe it probably had olive oil and a sprig of rosemary in it. It probably made a lovely gift.

As to the color, it's a post-infused bottle. One way to make cobalt glass is to mix cobalt in with your molten glass. On the other hand, it's cheaper to sprinkle your mold with a little cobalt oxide. If you want color all the way through, you infuse all the glass. If you want color on some of it, you infuse it. FUN FACT: Both rubies and sapphires are a mineral called corundum, which is chemically identical to rubies and sapphires except it's water-clear. Rubies and sapphires are obviously worth a lot of money while corundum is worth fuckall. However, you can take your corundum, facet it, polish it, then bake it at 4,000 degrees in a bunch of cobalt oxide and it'll come out rough but pretty, pretty blue. The blue layer is a couple mm thick, tops, but your worthless corundum just became a $200 sapphire. Dope it with chromium instead and suddenly it's a ruby. And while jewelers can usually tell, and while the GIA will tell you that the stone is worth barely more than a corundum, something like ninety percent of all rubies and sapphires sold on the retail gem market are doped. A lot of 'em have fuckin' glass in 'em these days.