In a well-cut faceted gem, the pavilion facets (those on the lower half of the stone) should reflect light back out of the top (table) of the stone. If the facets are cut below the critical angle for the particular material, light will pass through the stone instead of being reflected back towards the eye. When this happens, the gem will lose much of the sparkle and brilliance. Try holding your gemstone over a piece of paper with some text on it. Looking down through the table of the gem, if you can clearly read the text through the stone, it has a window. Almost all gemstones will exhibit some degree of windowing, but the lesser the better.
In addition, all gemstones will exhibit what is referred to as a 'tilt window'. Even the most well-cut stone will have areas that appear colorless when viewed at certain angles. It should also be noted that when it comes to colored stones, color is still the most important factor. Buying windowed gemstones isn't always bad, especially for opportunists. It can be a great way to save money if color or carat weight are more important to the buyer. And for those who have access to gem cutting services, buying a large windowed gemstone and having it recut can result in phenomenal savings.
Bought my first set of gemstones of gemselect to start off my collection. Everything is what it says it is, 100% legit. They even send u a pamphlet for every gem you order even the $1 ones lol. Good store.