Rule #1 it better be a style the girl wants. Most feel very uncomfortable discussing this with the boy or his parents.
When my eldest son was at the ring purchasing point, his two closest in age sisters had a ring shopping date with his future wife. they went to a few stores and the intended tried on different styles and sizes and discussed what she liked with my daughters. They even took pictures of her top choices on their cell phones.
Then this information was shared with my son, wife and myself,
Rule #2 Poppa will Pay….therefore Poppa (or momma) is involved in the actual purchase.
We went to a diamond merchant we have used over the years and chose a stone of suitable size, cut and quality that met my expected expenditure and showed the merchant a picture of the chosen ring. He called in a jeweler in the building and that jeweler provided the ring and the stone was mounted.
Remember, this ring will be worn for many years and what may be suitable on a 20 year old hand is not on a 60 year old hand. Don’t go trendy.
Rule #3 Stay away from anyone who offers to save you the tax by you paying cash. If the merchant is willing to cheat the government, he might also be cheating you