An 16.40 carat natural, faceted tricolour tourmaline from Afghanistan is a substantial and impressive gemstone. Given the size, the color zoning (tricolour effect), and the rarity of Afghan tourmaline, this stone would be considered a significant piece for collectors or investors in gemstones.

The value of such a gemstone would depend on several factors:

• Color intensity and zoning: The more vivid and well-defined the color bands (typically green, pink, and yellow or similar combinations), the more valuable the stone. A balanced distribution of the colors can increase its appeal.

• Clarity: Tourmalines often contain inclusions, but higher clarity (fewer inclusions) can raise the stone's value.

• Cut: The faceting of the stone plays a significant role in maximizing its brilliance and showcasing its color zoning. A well-cut stone will attract more attention and command a higher price.

• Origin: Tourmalines from Afghanistan, particularly those with tricolour zoning, are rare and highly sought after, which adds to the value.

In general, an 16.40 CT tricolour tourmaline with good color zoning and clarity could be worth several thousand dollars, with prices varying based on market demand and the specific attributes of the stone. If you're looking to sell or evaluate the stone, a certified gemologist or appraiser could provide a more precise valuation based on its specific characteristics.