Top 10 Gemstone Characteristics
Very often it's hard to make a choice because all these gem characteristics are desired but they may not be available in one gem - Color vs Hardness, Size vs Shape, and so on (what to buy? The big squire emerald or the small heart-shaped diamond?)Available lists on TheTopTens about gem colors :
Top 10 Gemstones with the Most Incredible Colors
Top 10 Gemstones with the Most Unique Color-Givers
Top 10 Gemstones with Two or More Colors Occurring in One Single Crystal
Top 10 Gemstone Varieties of the Mineral Beryl
Hardness is the ability of one mineral to scratch another and the harder gems are more resistant to scratching.
Diamond, sapphire and ruby are very hard and this is one of the reasons they are loved. You don't wanna spend a lot of money on a beautiful piece of jewelry that gets scratched way too soon.
list - Top 10 Extremely Hard Gemstones
List - Top 10 Best Shapes For a Gemstone
Lists:
Top 10 Gemstones with Phenomenal Optical Effects
Top 10 Gemstone Special Effects Inherent In the Mineral
Durability is different than hardness - durability is a combination of characteristics where hardness is very important for durability but isn't the only factor. For example diamond is the hardest gem but it is NOT the toughest gemstone - Jadeite Jade is.
Durability means resistance to different processes:
1) resistance to scratching (hardness) - hardness is the ability of one mineral to scratch another
2) resistance to cracking (toughness) - ability to withstand mechanical shock, like, bumping it against something (diamond is not the best at that)
3) resistance to chipping (emerald is pretty resistant to scratching because it's hard but it is very susceptible to cracking and chipping)
4) resistance to abrasive substances
5) resistance to dirt, and so on.
Cut and shape are closely related but not the same - sometimes overlap but not always.
List - Top 10 Gemstones that Are Always or Often Left Unfaceted (Cabochon)
List - Top 10 Sparkling Gemstones
Lists:
Top 10 Very Rare Gemstones
Top 10 World Famous Individual Gemstones
Clarity criteria is mostly applied to diamonds but in general it is a desired feature of most gems. However, some gemstones are valued for their inclusions and not clarity - for example, rutile "silk" inclusions in sapphire create the star in Star Sapphires and that star is highly desired (list - Top 10 Gemstones with Desired Inclusions).
Also, some minor impurities prove that the gem is natural and not fake.