The outside of quahog shell gives no clue to the  amount of purple inside the shell. Jewelry-grade quahog shells have been rare since the days of Roger Williams.
Here's why:
Examples of variability in quahog shells. Color: Like sapphires and rubies, value is all about color. Some quahogs have no purple at all. Most shells have a little purple on the inside surface, but the color does not go much below the surface. A few quahogs have quite a bit of purple that goes deep into the shell, sometimes with nice banding. And a few very rare shells have intense purple that extends deep into the shell. These are the most desirable ones for making jewelry. The amount of color is partly due to genetics, and partly due to the minerals in the mud or sand where the quahog grows.
Shell with tunnels made by Cliona the boring sponge. Freedom from Flaws: Just like diamonds, the value of quahog shell depends on its freedom from flaws. A common flaw results when the shell becomes a host for a boring sponge known as Cliona, who sets up housekeeping in a quahog by creating tunnels where she lives in a protected environment. Cliona and the quahog can live together just fine, but the tiny burows make the shell completely useless for jewelry.

There are other types of flaws, but the botom line is: jewelry-grade shell is far from plentiful. Adding to the scarcity, the demand for stuffies uses virtually all the shells opened commercially.

Sections of quahog shell show depth of purple and 
	  process for making wampum Why purple wampum was always worth more money: It takes thick shells to make wampum beads, and you can only use a narrow strip, where the shell is the thickest. (See box drawn on shell.) The rest of the shell is scrap. Even though that section can be solid purple on the top, the color may only be on the surface. As a result, most beads are a combination of purple and white. After the Europeans arrived with trade goods, the Indians used a steel needle mounted on a stick (rather than a sharp stone) to drill the length of a bead. This was an extremely time-consuming process, and they made strands of beads by the fathom (about six feet long).