If she is close to her friends or sisters, they likely know her exact preferences.
Notice what jewelry she wears daily (gold vs. silver, minimalist vs. flashy) to avoid buying something she won't wear, as seen in these Reddit discussions .
A durable, everyday ring (like a 2mm+ band) is often better than a "show piece" that might be too fragile, as discussed by Reddit users .
If you are unsure of her exact taste but want a surprise proposal, buy an inexpensive, simple ring and tell her you will go together to pick the final one.