Thanks in advance,
On several occasions, I’ve noticed I could find the zoning lines of natural corundum, rubies and sapphires, more easily with a polariscope. Sometimes, these lines aren’t easily visible under a standard 10X loupe. If you have a polariscope, try it. It may work for the curved striae in synthetic ruby and sapphire. If you don’t have a polariscope, you can make a cheap one. And don’t be afraid of using your loupe between the polarizing lenses.
A polariscope helps you locate the C axis of a crystal rather easily. If you use a polariscope to locate the C axis of a synthetic ruby, for example, then at least you’d expect the lines to be curved if you see them while looking down that axis. If you don’t look down the C axis, you may see straight lines that are actually curved. (Especially if you look at them at an angle perpendicular to the C axis).
Additionally, Richard Liddicoat recommended immersing a stone in methylene iodide or bromoform in a jar. This makes it easier to view the striae while using a white sheet of paper under the jar as a background.
Ryan Young (Hobbyist Gemologist/Mineralogist and Enthusiast)