The least abundant of the eight rock-forming elements that make up most continental crust is Sodium (Na).

Here's a breakdown of the eight major rock-forming elements in order of abundance:

1. Oxygen (O)

2. Silicon (Si)

3. Aluminum (Al)

4. Iron (Fe)

5. Calcium (Ca)

6. Sodium (Na)

7. Potassium (K)

8. Magnesium (Mg)

While sodium is a key component in many minerals like feldspar, it has a significantly lower abundance compared to the other elements listed.