The Carat is a weight measure used for gemstones. One carat equals 200 milligrams. The stones are measured to the nearest hundredth of a carat. Ruby and sapphire are heavier than diamonds, so a ruby or sapphire carat is smaller in size than a diamond carat. This unit of mass was adopted in 1907 and can be divided into one hundred equal parts of 2 mg each. Interestingly, if this measure is applied to gold it ceases to be a mass measure and becomes a measure used to assess its purity. For example, 18-carat gold has 75% gold, the other 25% is made up of other metals that are added to the part to make it look brighter and longer.
The Gram is a unit of mass and is one-thousandth of the standard kilogram. Although the kilogram is the fundamental unit of mass, in practice we use gram as the main unit. This is because, the multiples and submultiples of the mass units, are defined from the gram, see the case of hectogram, decagram, decigram, centigram, etc. Did you know that a cap of a pen weighs about a gram and that this is also the approximate weight of a paper money?