The green crown looks like the one used by Nippon:

I may be wrong but the base of the cup looks more like pottery than china. From the thickness at the lip's they may not be of the same set.

I would think it's porcelain.

Yes, they are from the same set with the same backstamp, and definitely porcelain and not pottery.

Thank you, I've been looking at the Nippons. The Nippon crowns are similar, but not exact. My china backstamp is a detailed green crown, with no other words, letters, numbers or symbols. Very frustrating! It could be bone china - it is very white and translucent.

In china marks similar doesn't mean a thing. One green crown is not like another just because it's green.
May very well be bone china.

Thanks to all for the searching. No match yet, but I'm not giving up so easily!