SAPPHIRE are the same hardness as ruby, both are Corundum, but sapphire is blue, light to dark, but can also goes to pink and to orange. When said only saphire it’s blue, and when it’s a other colors we say for example a pink sapphire.
INVISIBLE a perfect setting for only taking advantage of the stone.
FLAT is nice and bold, the perfect choice for a contemporary ring.
ROUND make the sides like vanishing, become a radius, smooth and comfy. In both cases, flat or roundish sides, to provide comfort, we break the edges by making a radius,