Good lead. I could not find ingredient sources (it is not mandatory in NZ to list those) but I did find this review: It appears the magnetic component is iron oxide, as suggested by @huutistuutis

Too many ingredients I don't want - they are red in colour, however Blackmores were helpful published 3 months ago I bought these Blackmores Vit D3 on sale at Chemist Warehouse, but found they were red in colour as compared with the usual clear coloured capsules. It was only when I looked up additional ingredients in a TGA public summary that I found out why. Here is a list of the other ingredients: Gelatin, glycerol, iron oxide black, iron oxide red, lecithin, medium chain triglycerides, mixed (low-alpha type) tocopherols concentrate, purified water, Soya Oil.

Compare this with the list of other ingredients in the Swisse Vitamin D3: Gelatin, glycerol, purified water, Soya Oil. Much simpler.

I tried to take the Blackmores product back to Chemist Warehouse, but this is a waste of time as they don’t answer their phones or reply to emails.

However, I was offered contact with Blackmores advisory service and was offered a refund. I have arranged this via reply paid with the product back to Blackmores. So I made it a 3 star review overall.

https://www.productreview.com.au/listings/blackmores-vitamin-d3

I beg to differ. https://pubmed.ncbi.nlm.nih.gov/34186257/