Hello Michael, From 3 mm diameter on, you can make in titanium
tension rings. I have also 4mm ones and I always say with these, if
people ask if the stone will not fall out, If you can remove the stone
with your fingers you may keep the ring. Of couse this will never be
the case. You need some reverse pliers for this job. Seegering pliers
will do perfect. Ask in an car tool shop for these type of pliers.
Greeting from Zwolle (Netherlands)