Not necessarily, an engagement ring should be made of Diamond. It can be gold or pearl as well. Unless, diamond is your favorite stone, an Ethical Engagement Rings should be something personal. Do not go for a ring, just because it is socially expected, you should choose a ring that is completely based on your personal choice.

Most importantly, if you are looking it for your girl, then it is a matter of taste what she likes. If she likes diamond, then it should be a diamond ring else it could be anything depending on her liking.

I would recommend you to have ethical designer rings (made of stone she likes) to make her surprise by presenting it on the engagement day.