Diamonds and Rubies and Sapphires and Emeralds...

In modern usage the precious stones are diamond, ruby, sapphire and emerald, with all other gemstones being semi-precious. There is no universally accepted grading system for gemstones. Aside from the diamond, the ruby, sapphire, emerald, pearl (not, strictly speaking, a gemstone) and opal have also been considered to be precious. Gem prices can fluctuate heavily (such as those of tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices. Typically prices can range from US$1/carat for a normal amethyst to US$20,000–50,000 for a collector's three carat pigeon-blood almost "perfect" ruby.
Adapted from http://en.wikipedia.org/wiki/Gemstone- see http://creativecommons.org/licenses/by-sa/3.0/ for license information.