How to read the band →
Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Opal
Opal has been climbing in recent years. All-time it's been given to 74,257 U.S. babies, the 882nd most common name by sound today, peaking at 2,886 in 1918, with 740 in 2025.
Opal has no common alternate spellings by sound.
Rising
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).