If you do not have Comprehensive coverage on your vehicle than normally your windshield being broken by a rock kicked up by another vehicle would not be considered a hit and run thus not covered by your Uninsured Motorist Property Damage coverage. Glass breakage on your own vehicle would also not be covered by your Liability coverages.
This type of "accident" would find the other driver not negligence in most cases (even if you knew who the car belonged to). This type of incident is typically termed a road hazard or an event that may happen when you are driving on the roadways. If instead you were following a commercial truck that was loaded with rocks or gravel and they were not tied down properly than you may have a case in which to prove negligence. By proving negligence the other vehicle would then usually be liable for rocks that cracked your windshield but you would then need information on that vehicle.
When a passing vehicle kicks up a rock from the roadway that hits your vehicle and cracks your windshield then it would be doubtful that the other driver (if known) could be held responsible in any way since that was an unintentional road hazard.
If the passing car kicked up a piece of the roadway, such as a piece of pothole that the state Department of Transportation (DOT) had filled, then the DOT or state may be requested to pay. It would be hard to prove negligence on the part of DOT though just as it would be on the passing vehicle that happen to kick up a piece of gravel lying on the roadway. So while a request could be made with the DOT if it was part of pothole it could be difficult to show negligence and get them to pay out on a claim.
Since the other driver is unknown (and even if know would likely to not be found negligent), you would normally need to make a claim through Comprehensive coverage to get the crack in your windshield either repaired or replaced. As we mentioned earlier neither Uninsured Motorist nor Liability insurance would cover this.
It is unfortunate that you do not have Comprehensive coverage on your vehicle since South Carolina requires insurance companies to waive the deductible associated with windshield claims.
According to the South Carolina Department of Insurance (DOI) if you have the optional physical damage coverage of "other than collision" or Comprehensive then broken glass is covered. In SC Comprehensive coverage will cover broken glass, such as windshield damage, and the deductible does not apply in this case.
With only the state's minimum Liability insurance requirements and Uninsured Motorist on your car in South Carolina then your windshield would not be covered.