Do you know it's precise origin? Species, location, method of harvest etc.
Lab certification as natural would likely increase the value, but whether it's cost effective is something to consider. Likewise, if it's a keishi... you'd lose money.
Assuming it is natural, there is no certain criteria from which valuation can be established. We often say here, that beauty is in the eye of the beholder... and such is value.
Unless you harvested this pearl yourself, it's wise to be wary of "natural" claims.