Learn Polish: Twardość to bardzo ważna właściwość kamieni. - Hardness is a very important property of stones.

In Polish, when you define one noun as another (like 'Hardness is a property'), you have two choices. You can use to and keep both parts in the basic Nominative case, as we do here: Twardość to... właściwość. If you used jest, the second part would have to change to the Instrumental case: Twardość jest bardzo ważną właściwością. Using to is often simpler and very common for definitions.

Both twardość and właściwość end in -ość. What gender are these words?

They are feminine. Almost all Polish nouns ending in -ość are feminine. This is a very helpful pattern because -ość is the standard ending used to turn an adjective into an abstract noun, much like '-ness' or '-ity' in English (for example, twardy means 'hard', so twardość means 'hardness').

Why is the word for 'stones' written as kamieni?

It is in the Genitive plural case, which translates to 'of stones' here. The base dictionary word is kamień, which is a masculine noun ending in a soft consonant (-ń). While many masculine nouns take -ów in the Genitive plural, those with soft stem endings usually take -i or -y instead.

Why is the adjective ważna in the Nominative case if it is sitting right next to the Genitive word ?