Niek is not a diminutive of Nicolaas. It is actually a short form of the name, in the exact same way that Nick is a short form of Nicholas. [noted -ed] To visualise:• Nicolaas (pron. NEE-ko-las) ---> Niek (pron. NEEK)• Nicholas (pron. NIK-ə-ləs) ---> Nick (pron. NIK)If Niek were a diminutive, then it would have contained one of the Dutch diminutive suffixes, such as -je (i.e. the name would have been Niekje):https://en.wikipedia.org/wiki/List_of_diminutives_by_language#Dutch (in English)Also see this important link:http://www.meertens.knaw.nl/nvb/naam/is/Niek (in Dutch; mentions under "Verklaring" that Niek is a short form of both Nicasius and Nicolaas - the former primarily in the Dutch province of North Brabant)And compare these bearers:• Niek du Toit (born Servaas Nicolaas du Toit): https://af.wikipedia.org/wiki/Niek_du_Toit (in Afrikaans; he is known as Nick in English) • Niek Michel (born Nicolaas Johannes Michel in 1912): https://en.wikipedia.org/wiki/Niek_Michel (in English) • Niek van Dijk (born Cornelis Nicolaas van Dijk in 1951): https://en.wikipedia.org/wiki/Niek_van_Dijk (in English)With that said, there have been rare cases (mostly in the past) where Niek and the variant spelling Nieck were Dutch misspellings of the Frisian names Nyk and Nyck, which are both pronounced as NEEK. In Frisian, the 'y' is always pronounced as 'ee'. This is not the case in Dutch, hence the Dutch sometimes writing the Frisian 'y' as 'ie', which is the Dutch equivalent of the English 'ee' sound. By the way, the Frisian names Nyck and Nyk are also ultimately derived from the ancient Greek name Nikolaos, so etymologically, there is no significant difference from Niek.
• Nicolaas (pron. NEE-ko-las) ---> Niek (pron. NEEK)
• Nicholas (pron. NIK-ə-ləs) ---> Nick (pron. NIK)
If Niek were a diminutive, then it would have contained one of the Dutch diminutive suffixes, such as -je (i.e. the name would have been Niekje):
https://en.wikipedia.org/wiki/List_of_diminutives_by_language#Dutch (in English)
Also see this important link:
http://www.meertens.knaw.nl/nvb/naam/is/Niek (in Dutch; mentions under "Verklaring" that Niek is a short form of both Nicasius and Nicolaas - the former primarily in the Dutch province of North Brabant)
And compare these bearers:
• Niek du Toit (born Servaas Nicolaas du Toit): https://af.wikipedia.org/wiki/Niek_du_Toit (in Afrikaans; he is known as Nick in English)
• Niek Michel (born Nicolaas Johannes Michel in 1912): https://en.wikipedia.org/wiki/Niek_Michel (in English)
• Niek van Dijk (born Cornelis Nicolaas van Dijk in 1951): https://en.wikipedia.org/wiki/Niek_van_Dijk (in English)
With that said, there have been rare cases (mostly in the past) where Niek and the variant spelling Nieck were Dutch misspellings of the Frisian names Nyk and Nyck, which are both pronounced as NEEK. In Frisian, the 'y' is always pronounced as 'ee'. This is not the case in Dutch, hence the Dutch sometimes writing the Frisian 'y' as 'ie', which is the Dutch equivalent of the English 'ee' sound. By the way, the Frisian names Nyck and Nyk are also ultimately derived from the ancient Greek name Nikolaos, so etymologically, there is no significant difference from Niek.