Air Bubbles

Sometimes they have a similar color and relief to garnet, with a dark border.

Air bubbles looking a bit like garnet, plane polars

Sometimes they look like thick black rings.

Plane polars
Air bubble, XPL

Sometimes you get a plague of them.

A plague on both your houses thin sections!, plane polars

Sometimes you’ll see a mineral with a really interesting extinction pattern. Nope. Air bubble.