In addition to the 4Cs, gem values are influenced by natural rarity and the economics of supply and demand. This explains why gems that look similar in colour and size can differ substantially in price.

Both sides of the equation of supply and demand come into play. Alexandrite, rare but in demand from collectors, may cost as much as better-known gems. Some gems are relatively rare but since they aren’t well known, there is little demand, keeping prices low. The ancient world couldn’t tell ruby and spinel apart, but spinel’s role as an unknown understudy keeps its price relatively low today even though a one-carat red spinel is more rare than a one-carat ruby. Other rare gems that are relatively affordable include tsavorite garnet, morganite, red beryl, and zircon.

Within each gem variety, quality determines cost. But different gem varieties have different pricing structures. In general, classic ruby, blue sapphire, and emerald are the most expensive gemstones. One level below these are the rare collector’s gems like alexandrite, demantoid garnet, Paraiba tourmaline, black opal, pink topaz, jadeite, chrysoberyl cat’s eye, fancy sapphire and South Sea cultured pearls. The moderate price range includes tanzanite, tsavorite garnet, tourmaline, red spinel, aquamarine, precious topaz, spessartite garnet, and Tahitian cultured pearls. Affordable gemstones include amethyst, citrine, spinel, many colours of garnet, blue topaz, chrome diopside, fire opal, white opal, iolite, kuzite, peridot, coral, turquoise, lapis lazuli, chalcedony, and freshwater and saltwater cultured pearls.