In a matter of months the UK government is likely to introduce legislation that will restrict the sale, import or export of any (silver) item which has, by volume, more than 10% ivory (excludes musical instruments).  The detailed legislation is still to be worked out but I have built a simple spreadsheet you can plug in your silver item’s weight and measurements of ivory components to calculate whether the piece meets the likely de minimis rule.  If you would like a version please contact me.