It will be fun to see how the holes are "made" if you are able to get a shot.

"The holes are conical to the middle and meet a little offset" per OP Mark. Could be bow drilled material such as stone, bone, or fossil material perhaps? (Remember no expert here).

More guessing-I should think if it were pottery or powered glass that they could have inserted something like a reed and have it burn away to leave a central hole? So this makes me think otherwise.

So sort of guessing by the 5mm grid- about 55mm/ 0r around 2" ?

I added to my crazy list:
? magnetic
? radioactive (as in fossil bone FYI),

Cool!

Anne