Question: Is it scientifically correct to call Diamond an element? Carbon has a wide range of allotropes, so would it be correct to actually just call graphite, diamond etc an element. For that matter, could we call any molecule containing only one type of atom, an element?

Answer: 
**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."
Diamond is a form of *pure carbon* (which is an element). Logic may lead you to believe that diamond is, therefore, an element. However, this is a bit circular because if you say that diamond is an element, then every allotrope of every element also falls under the same consideration. 

Also, in order for diamond to be a unique element, the molecules that make it up should have a different number of protons than any other element on the periodic table. Since a diamond is made of carbon, it is really just another form of carbon.