The second most abundant element in the Earth's crust is silicon (Si). It is also the principal component of sand and quartz.

Here's why:

* Silicon in the Crust: Silicon makes up about 27.7% of the Earth's crust by weight, second only to oxygen (46.6%).

* Sand and Quartz: Sand is primarily composed of silicon dioxide (SiO2), commonly known as quartz. Quartz is a very hard and resistant mineral that makes up the majority of sand grains.

So, the answer to your question is silicon.