Calling Diamond an element?
It might be scientifically correct but it is linguistically misleading

The sentence "diamond is an element" can be seen to be misleading when compared to the sentence "diamond is an allotrope of the element carbon". Or even "diamond consists of the element carbon". 

The issue is that clear language should distinguish between the form and the composition of an element with more than one allotrope. Saying "diamond is an element" confuses the fact that diamond is made from carbon with the fact that it is made from an infinite tetrahedral array of carbon atoms. It also subtly excludes the fact that there are other allotropes. 

For clarity in language a specific form of an element (an allotrope) should not be referred to as "an element" but should be referred to as one form of that element. As in "diamond is one form of the element carbon, buckminsterfullerene is another form."