Combine the chopped Fresnos, grenadine and 1/4 cup cold water in a blender and blend until smooth. Pour into 3/4 cup cold water and stir to combine. Pour into ice cubes trays and freeze until solid, about 2 hours.
Combine the orange juice, tequila, triple sec, lime juice and agave in a pitcher and stir to combine. Add the halved Fresnos and muddle slightly with a wooden spoon. Chill at least 1 hour.
When ready to serve, combine the sanding sugars and 1/4 cup salt in a small, shallow dish. Dip the rims of 4 rocks glasses in water, then into the sugar mixture to coat the rims. Fill the glasses with the Fresno ice cubes and strain the margarita over the top.
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