Tahini natural separates if it has been sitting for a while. The oil will float on top and paste at the bottom will thicken and harden. This is normal and totally fine to consume! If this happens, don't discard that oil on top or you'll be left with dry, crumbly sesame paste. Instead, shake the jar with its lid tightly closed to help mix it all back together or use a fork to whisk. I've even scooped it all into a food processor to mix it up back up. This is the best way to restore it back to it's creamy consistency without any lumps!