Tip #1: Examine the Inventory
A quality rock shop should have plenty of gemstones to choose from, both cut and rough. While a shop may have cheaper stones in its collection, like quartz crystals, all of its inventory should be of at least good quality. The presence of inclusions and blemishes should be minimal, and the shop should be honest about the flaws that a particular stone has.
Tip #2: Talk to the Owner and Staff
Your gem dealer should be knowledgeable about the items that are sold. Most reputable dealers will go directly to a quarry or other rock source and hand select stones for their shops, though some dealers may have a trusted professional perform this service for them. Regardless of who chooses the stones, the entire staff of a rock shop should know plenty about the business. A good rock shop staff will want to match you up with a stone to suit your preferences, not whatever stone happens to be the oldest or most expensive. They should be willing and able to answer all questions you might have, from the qualities of tourmalines or amethysts to their countries of origin.
Tip #3: Inspect the Sales Policies
A reputable rock shop will stand behind its inventory. While businesses are not in the habit of insuring products indefinitely, your new gemstone should come with some sort of limited guarantee. A good lapidary or jeweler will be willing to periodically inspect your stone for flaws, and a free cleaning may be included, too.