diamond

No. Isotopes are refering to the different forms of an element each having different atomic mass. For diamond, it has no difference to carbon in atomic mass. Therefore it is not a isotope.

Allotropes of carbon : different forms of carbon.
Isotopes of carbon : atoms of carbon having different mass numbers.

Diamond is one form of carbon, so that it is an allotrope of carbon. In diamond, it contains different isotopes in similar proportions to other allotropes.