The noun jewellery is an uncountable noun for an aggregate, a word representing an indefinite number of parts or things. Multiples for the uncountable noun jewellery are expressed as pieces of jewellery, some jewellery, a lot of jewellery, etc.
The word jewelery is a non-count noun which doesn't have a plural form; units are referred to as a piece of jewelry or pieces of jewelry, an item of jewelry or items of jewelry, etc.
The plural form for the demonstrative pronoun this is these.
The plural form of the word inferno is infernos.
The plural form is ranches.
There is no plural word for if.
There is no plural form for the word, countries. This word itself is a plural.
The plural form is data.
The plural form for the word 'pioneer' is pioneers.
The plural form of the word "baby" is "babies."
The plural form of the word "pharynx" is "pharynges."
The plural form of vow is vows.
The word jewelry is a mass (non-count) noun. Multiples are expressed as a little jewelry, a lot of jewelry, a quantity of jewelry, a selection of jewelry, etc.