The phrase "a core of which" is correct and usable in written English.
It can be used to refer to a central or essential part of something previously mentioned, often in a more complex sentence structure.
Example: "The organization has several initiatives, a core of which focuses on environmental sustainability."
Alternatives: "the essence of which" or "the main part of which".
Most of the volunteers come and go under their own steam, but some stick around, and there is a core, of which Burton and Colquhoun are part, whose accommodation and living expenses are paid by Help Refugees.
This scenario analysis suggests a more serious study of these proposed implications for the kind of American "Exceptionalism" -- a core of which is our longstanding commitment to birthright citizenship -- which really is an exception compared to most countries, especially those in Europe.
Hume, though far from a romantic figure, had a core of courage, which both impressed Smith and frightened him a little.
Underneath Ms. Allyson's sweetness is a core of toughness, which revealed itself in the Blossom Dearie hit "Bye-Bye Country Boy," about a traveling musician's dalliance on the road.
"There's been this streak of creativity and once you got a core of people, which started round 2009, you started to get a lot of investors from the US and Europe looking to Berlin.
Then we saw a gadget, the core of which was a snow shovel but had added grips and lerers so that the snow could be lifted automatically.
But there is a world of difference between selling someone a meat pie for their lunch, and being asked to devote artistic talents to the "celebration" of a relationship, at the core of which is a rejection of a Biblical view of appropriate sexual behaviour.
The coring-type diamond bit makes an annular hole, the core of which provides a sample cross section of the strata penetrated, and is used for prospecting.
The alt-right movement, as described by the Southern Poverty Law Center, is a group of people who subscribe to a far-right ideology "at the core of which is a belief that 'white identity' is under attack through policies prioritizing multiculturalism, political correctness and social justice, and must be preserved, usually through white-identified online communities and physical ethno-states".
This multiple representation principle is based upon the Cognitive Theory of Multimedia Learning, a core principle of which is the dual channels assumption, which proposes that learners process information through separate auditory-verbal and visual-pictorial channels [ 25, 26].
That is, we used a statistical method, at the core of which was hard data on prevalence for a limited number of countries, plus data on factors that are relevant to vulnerability to enslavement.
