What to look for when hunting gemstones (sapphires)

Found an area close to home that has had sapphires and gold pulled out of it in the past. I am pretty confident I'll find something there. When I have found sapphires in the past I just treated it like am looking for gold. I know once I hit black sand I am on the right track with gold. I found black spinel everywhere I have found sapphires, is there spinels everywhere there are sapphires?? What other indicators should I be looking for? Is there any geographical features that I should particularly looking for?

I would say spinel are the best indicator for sapphires. Just look for the gravel layer , which may only be an inch or less thick