The discussion above is accurate as far as it goes, which is only to the first significant figure. To go any further one needs to be relatively sophisticated about surveying the monolith (including realistic and explicit assessment of the shapes of inaccessible portions of the monolith), then about calculating the volume (and volumetric errors, which vary crudely as the cube of linear uncertainties). Finally and crucially the rock density needs to be measured with appropriate precision. Identifying the rock type is not going to be sufficient as this table (from) illustrates:
Simply identifying the monolith as being "sandstone" would allow a reasonable ± 15% uncertainty in the weight estimate. In practice, one would measure the density of the monolith itself, and preferably document any variation in density within the monolith as they are made of natural materials, which have not been engineered for homogeneous parameters. Non-destructive methods of density measurements are available (e.g. electron back-scatter); alternatively the site may contain already-separated fragments of the monolith which can be used for laboratory measurements or on-site techniques. At the crudest, a weighing device and a bucket can obtain two significant figures for a density value.
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