white gold art deco ring with no hallmarks??(expensive)?

hi, my sister has just bought a white gold,old art deco ring, (with small diamonds and a small sapphire).. she got it from a exclusive jewellers, (she had just got a inheritance, and she choose this ring as a keepsake) she paid a LOT of money for it!!
my concern is that there is not any hallmarks on it, but I thought by law there had to be hallmarks, was this law in place in the art deco period? (1915-1935- ish) she does have a valuation certificate, but am i just being overly suspicious??.. she can’t afford to be ripped off for that kind of money..
I have (tentatively) advised her she needs to get it independently valued, (insurance purposes) but can anyone put my mind at rest please?
thanks x

There are a few reasons it may not have a mark:
it could have worn away
the ring could have been made smaller and sometimes not brilliant repairs can obliterate the mark
Are you sure its white gold? Platinum (more valuable) would have been a popular choice for the Deco period. Platinum would not have been hallmarked at that time, not until 1975 (UK)
It may have been made abroad and although it should have been hallmarked if imported it may have been bought by a person not a jeweller ad entered the country that way
(Always remember that reselling jewellery never gets half the shop price )
If she loves it, try and love it too, it will make her happy

Diamond Art Deco
Diamond Art Deco
Diamond Art Deco