Ornaments are of two types: ornaments of gold and silver, and ornaments other than gold and silver.

1- Ornaments of gold and silver.
The first division: ornaments intended for storage and safe-keeping, or the ones considered for business. Zakah on these is obligatory.

The second division: ornaments intended for use, the safest view is to pay the Zakah of these ornaments to free the conscience of guilt. A woman came to the Prophet ﷺ in the company of her daughter and on the daughter’s hands were two heavy Masakats[ Two masakats: two bracelets (bangles)] made of gold. The Prophet ﷺ said to her: “Do you pay the Zakah on these?” She replied: “No” He said: “Will you be pleased if Allah makes for you (from them) bracelets of fire on the day of resurrection?” She removed them and threw them towards the Prophet ﷺ saying: “They are for Allah and His Messenger.” [ Source: Abu Dawud]

There are some scholars who do not view Zakat on ornaments (meant for use) as being compulsory, because such an ornament is not a property kept to increase wealth, but a personal belonging to be used and benefited from like clothing, furniture and utensils; it is part of the need of a lady for her beautification, and the basic rule is that the wealth should actually increase or have a tendency to increase before Zakah is payable on it.

The safest course of action however, is to pay Zakah on ornaments meant for permissible use and beautification, because this is the safest opinion as it frees the conscience from guilt. The Prophet ﷺ said: “Leave that which you are in doubt of for that which you doubt not.” [ Source: Al-Bukhari]

2- Ornaments other than gold and silver
Zakah is not obligatory on diamonds, rubies, pearls etc., no matter how much its value is, except for the ones considered for sale, as in this case they are to be treated as goods displayed for sale (merchandise).
source: islamkingdom