Are you thinking about Beryllium?
Inhalation of the dust is harmful to your health.
Sometimes used in microwave ovens ... various other electronic applications. Beryllium - Wikipedia, the free encyclopedia
I may faintly recall a forum post from a member handling neodymium magnet filings more than beryllium, falling ill for a scary long time with hard medical times to decontaminate. Post is around a year old or less.
Tried full google and ojose searches with varied terms to not exclude other forums in case it was not here, with no success. The title of the post may not reflect well the contents, increasing misleading.
Noticed the search feature here provides very different results when being or not logged-on.
What am sure of, it was a discussion on a forum, and was rare earth magnet filings, and a scary long illness, describing symptoms and progress.