All gems can be scratched by anything of equal hardness or harder than. At 6 1/2 to 7 1/2 some garnets can be scratched by quartz at 7 hardness. Quartz is common in nature. Those over 7 are less likely to scratch. Other rings worn next to the garnet like diamond, sapphires and rubies, beryls and other harder gems can be a source of scratching. Cracking is not common.