We recommend inspecting both graded or ungraded diamonds before going ahead with purchase. This avoids any issues later on post purchase.
Some graded diamonds can be bought sight unseen, we don't recommend that for any ungraded diamonds.
Here are some reasons why diamonds may not be accompanied by a grading report:
The seller may not have graded the diamond because of cost (size/low color etc...). In that case, you can negotiate with the seller to make purchase conditional on grading or inspection.
Some sellers have grading reports that we do not recognize on RapNet and so they list the diamond as ungraded. This could be because there are very few reports of that type or RapNet doesn't recognize for quality reasons. In that case, you need to make a decision after discussion with the seller as to whether the report is sufficient for your purposes.
If it is economically viable, RapNet can inspect diamonds in India, Hong Kong, New York and Israel. There is a cost involved and it will vary depending on the services performed.