Outputs from crystal quartz may be sine, but usually crystal oscillators outputs square wave, 50% duty cycle. Another reason for which you may see such high freq. as sine wave may be the bandwidth of your oscilloscope.
Crystal oscillators around 100MHz may come in ACMOS/HCMOS which
will give about 3ns rise/fall time. PECL may give you about 2ns.
You might say these are not "square", but how square do you want?
And why?
Not many people have access to test equipment to check the "squareness"
in this range.
The most important thing may be the pupose of specifying "square wave"
in the first place. Jitter may be also important.