Top 10 Gemstones that Are Always or Often Left Unfaceted (Cabochon)Faceting is usually applied to transparent stones with a hardness higher than 7/10 on the Mohs hardness scale. Softer gems get scratched easier but scratches show much less on an unfaceted than on a faceted stone.
Opaque gems, softer than 7/10 gems, gems with special optical effects and inclusions are always or often just shaped and polished as opposed to faceted. The resulting form is usually a convex (rounded) obverse with a flat reverse. These gems are called cabochons (aka cabs). A cabochon may be cut in any shape, though oval is the most common.
Cutting "en cabochon" (French) means "in the manner of a cabochon".
The Top Ten
Rubies are very hard and ordinary rubies without a star are faceted. But the rarer Star Rubies (3% of all rubies) are not because you can see the star better without facets. - Metal_Treasure
Sapphire is the 2nd hardest gem after diamond but Star Sapphire is always cut "en cabochon" - the reason is to enable seeing the star better. Facets would ruin the star effect. - Metal_Treasure
All of these look awesome. - PhoenixAura81
Opal is always cut "en cabochon" - to keep the play of light effect and to conceal possible scratches because opal is pretty soft, only 5.5-6.5 /10. - Metal_Treasure
Awesome gemstone - AliciaMae
The 3rd hardest stone but it's not faceted in order to keep the Cat's Eye effect. - Metal_Treasure
Lapis Lazuli is actually rock with a nice deep color and golden shimmer. - Metal_Treasure
I thought that said Logan Paul for a second
This is a very pretty rock. I love it’s blue color. - AliciaMae
Moonstone looks beautiful! - BlueTopazIceVanilla
It's always cut "en cabochon" - Metal_Treasure
Looks like polished wood. - Not_A_Weeaboo
I absolutely love this gemstone!
Not always cut "en cabochon" but often - the most important thing is to see better the beautiful rutile needles inside. - Metal_Treasure
Because it's soft and has objects inside, such as plants and insects - Metal_Treasure
*Jurrasic Park flashbacks* - Not_A_Weeaboo
It's cut "en cabochon" because 1) it's very soft, and 2) you see better the patterns resembling angel's wings and feathers. - Metal_Treasure
It's always cut "en cabochon" - it's soft and has objects inside that must be visible - Metal_Treasure
It's soft and opaque so no point to facet it. Just polish it to show its unique colors. - Metal_Treasure