The law requires your employer to make at least one 10 minute break available for every four hours of work. The break must be a continuous, uninterrupted 10 minutes. Although the employer can require you to remain on the premises during the break, you cannot be assigned any duties or asked to remain on call.
I am not sure what you mean by "they have chosen not to take a lunch break". An employee can usually voluntarily choose not to take a meal break (or a rest break for that matter). However, an employer must truly make certain its employees are permitted to take at least a full 30 minutes continuous, off duty meal break every five hours (unless your shift only lasts six hour in which case you can waive the meal break). Many times employers argue that employees "choose" to work through meal breaks but in reality the employees are kept too busy or given way too much work to really permit them to take the break.
The employer can be required to pay you one hours wage for each day you are not permitted to take a timely meal/rest break of the proper duration.
You can review the DLSE website and the DLSE opinion letters for more details about these requirements.