How lovely for you to have your Grandmother's pearl necklace. Here are some guidelines for the information we need to evaluate your pearls. You have answered some of these questions already, I know.
1. Do the pearls feel slightly gritty or smooth when rubbed gently against your tooth or against another pearl?
If they are gritty, they are likely to be real nacre (genuine pearls.) If they feel smooth, they are likely imitation pearls.
2. Please provide clear, in-focus photos without flash against a white background (a paper towel works nicely.)
Include close-ups of the clasp (front and back) and a few of the pearls. If there are flaws, include a photo of those. Also the box they came in, and tags if you have them.
3. Any history you can give us about the pearls. Where/when you or your relative got them, any documentation you have (receipts, appraisals), their price range if you know it, etc.
4. Describe any marks on the clasp. These may be numbers (14K, 585, 750, 925 etc.) or brand names or even pictures.
5. Measure the pearls, with a millimeter ruler if possible. If they are graduated, measure the largest and smallest pearls.
Please take photos of the jewelry box also, and good clear photos of the pearls. We're looking forward to seeing your treasure!