@depthless for just a little more info, this is from the Help page on creating Custom Fields:

You can index the field for global searches if Store Value is checked, and if you have chosen any of the following in the Type field: Currency, Decimal Number, Email Address, Free-Form Text, Help, Hyperlink, Inline HTML, Integer Number, Percent, Phone Number, Text Area.

You cannot index a custom field for global search if None is selected for any Level for Search/Reporting option on the Access subtab of the custom field record.