Hallmarking Exemptions
Only jewellery over a certain weight limit legally requires a hallmark - over 7.78g for silver and over 1g for gold.
Sending jewellery to an assay office for hallmarking incurs a cost, so most jewellers only send jewellery that exceeds the weight limits, to keep the cost down for their customers.
For silver jewellery under the weight limit, jewellers tend to stamp '925' on these items to indicate it has been made from sterling silver. This is not a legal requirement, but tends to be reassuring to customers.