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."
The IUPAC definition of element is as follows:

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> A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. [IUPAC Goldbook definition of element](https://goldbook.iupac.org/terms/view/C01022)
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This seems to allow diamond to be called an element. But it's not a good idea, scientifically or linguistically, since it doesn't make sense.

Carbon is an element. Diamond is a *form* that the element carbon can take.

Let me give you a car analogy:
Toyota is a brand of car. The Camry is one example of a type of Toyota car. I.e., it is one form a Toyota can take. That does not make "Camry" a brand of car, nor would it make sense to call a Camry a car brand.

In summary, referring to diamond as an element, rather than a form an element can take, is like referring to Camry as a car brand, rather than as one of the brand's models.