Employers are required by law to allow for certain forms of religious expression at work, and to make "reasonable accommodation" for religious practices, including displays of religious symbols. Unless doing so creates "undo hardship" for the employer, an employee can wear jewelry or clothing that expresses their religion, as well as display religious symbols in their personal workspace.
It's against U.S. federal law for an employer to discriminate against an employee because of his or her religion. The manager must make a reasonable accommodation for the workers' specific religious practice. For example, if the employee's faith requires that they pray or meditate every day at noon, the manager should make a reasonable effort to provide a quiet space for the worker to do so. If the worker's religion requires that they wear tattoos or religious symbols, this right is protected.
In the United States, government agencies must demonstrate separation of church and state. This means not showing a bias toward any one religion. If a government worker engages with the public, or if their workspace is visible to the public, it must be clear that the religious beliefs are that of the individual employee, and not an endorsement of one religion over another by the agency. In most cases however, the employee can still wear religious jewelry or display religious symbols at their desks.
Display of Symbols
An employer cannot demand that an employee remove religious jewelry from their person or religious symbols from their workspace, unless displaying these symbols would create an "undo hardship" for the employer. This generally refers to safety concerns. For example, if a company bans wearing rings that may cause a hazard when working with machinery, the employer can ban rings with religious symbols, as well as all other types of rings.
Employers who want to create a workplace that welcomes diversity should act with sensitivity. Toward that end, unless the employer has a specific religious affiliation, it's better to avoid the appearance of favoring one religion over another. Year-end celebrations can include what the U.S. Supreme Court calls secular symbols, such as Christmas trees and Santa Claus. Other secular symbols include shamrocks, the Easter Bunny, and Easter eggs.