Nope. Red diamond is the most expensive. There are less than 20 natural red diamonds for sale on Earth at the moment.
Since the Koh-i-Noor is not for sale, it cannot be called the most costly diamond.
There are several reasons why a natural, gem-quality diamond is costly, the foremost being it's rarity. Only about 20% of all diamonds mined are gem-quality. Also, lab-created diamonds are less costly that natural diamonds. Industrial quality diamonds are not costly.
A white buoy with orange bands and a orange diamond is a hazard warning, you should stay away from this area.
A white buoy with orange bands and a orange diamond is a hazard warning, you should stay away from this area.
A white buoy with orange bands and a orange diamond is a hazard warning, you should stay away from this area.
Danger
A white buoy with orange bands and an orange diamond is a hazard buoy. Watch out for shoals, rocks, fallen trees, construction, or other hazards.
Vertical white with an orange diamond, and orange bands above and below the diamond,
A white buoy with orange bands and an orange diamond is a hazard buoy. Watch out for shoals, rocks, fallen trees, construction, or other hazards.
The superlative for costly is "most costly" or "costliest."
It's impossible to tell the largest diamond ever used in an engagement ring, but the most costly was Beyonce's at $5 Million, given to her by Jay-Z it had an 18 Carat diamond at its centre.
Luvaglio is the most costly laptop. Its hand made.