BORT:

From Merriam Webster online: imperfectly crystallized diamond or diamond fragments used as an abrasive.

So, industrial diamonds, or diamonds not worth making into jewelry.

MW says the word is likely from the Dutch ‘boort’ and dates at least to 1622, post period for my SCA work. (which is pre 1600)

Yes, I expect you’d like to know how stones were cut prior to then, right?  Keep tuning in.  I’ll blog about it one day.  Maybe this week.