both form are used
like 24-Karat (Carat)

The English translation of «карат» is “carat”, which is a unit of weight in gemstones. One carat is about 3 grains of troy, or avoirdupois, weight. (Possible abbreviation: c. or ct.).

Although very similar in spelling, the other English word, “karat” (Possible abbreviation: k. or kt), is used specifically in measuring the fineness of gold, with pure (100%) gold equaling 24 karats. In this meaning, to denote gold fineness, the word “carat” can also be applicable.

In other words, ct. is more often used with gemstones (e.g. a 0.5 ct. diamond), whereas kt. is more common to describe gold (e.g. 18 kt. yellow gold studs).

In your example, the ring seems to have an exceptionally large diamond (a 56 ct. diamond ring?), since, clearly, 54 cannot stand for gold fineness.

See how these English abbreviations are used at http://www.mondera.com/index.asp?sourceid=MK0BRT7H00

Regards.

In English, carat has two different meanings:

1. A unit of weight in gem stones (200 milligram, or about 3 grains of troy or avoirdupois weight).

Russian:
- карат, мера веса драгоценных камней (200 мг - метрический карат; 205 мг - британский карат).

2. A unit for measuring the fineness of gold (pure gold being 24 carats fine).

Russian:
- карат, мера содержания золота в сплавах, равная 1/24 массы сплава.

Spelling variant: karat

Etymology:
Keration (in Greek, little horn), carob tree bean (стручок рожкового дерева Ceratonia siliqua, плоды которого служили мерой массы), weight of 3 1/3 g.

Have a nice day!