To follow up on Muezenhamster's comment, given the apparent presence of a sprue consistent with a gang mold, the round object does present the appearance of a musket/rifle ball.
Some additional questions:
Given the apparent slight irregularities in the diameter of said ball, how was the 13mm measurement taken? Also, to what degree of decimal accuracy was the scale utilized in the weight measurements?
Reasons for the above questions:
13mm = .51 caliber. An average lead ball of this diameter, depending upon purity and entrapped micro-bubbles will weigh ~ 177 grains (Avoirdupois).
177 grains = 11.46 grams.
13 grams = 200.616 grains.
Thus you would appear to be presenting an object with a density greater than lead. The lack of significant oxidation combined with the coloration would be rather consistent with lead.
It may be possible that more accurate metrics can resolve this matter.
The objects were found in eastern Spain (Mediterranean), in an area owned by my family on the mountain. There were underground and that area is not archaeological, nor is there archaeological remains of ancient civilization.
The measuring instrument to calculate the weight I used is not very accurate, I have no other. But I tried to gauge the weight using different euro coins in order to know the possible error of weight. These are the results:
Weight as a measure obtained using the gram:
1 Euro coin (€):
official weight: 7,5 grams
Weight measured 5 times: 8 grams
Weight measured 1 times: 7 grams
1 50 cents € coin (€):
official weight: 7,8 grams
Weight measured: 8 grams
1 20 cents € coin (€):
official weight: 5,74 grams
Weight measured: 6 grams
1 10 cents € coin (€):
official weight: 4,1 grams
Weight measured 5 times: 4 grams
Weight measured 1 times: 5 grams
Weight as a measure obtained using lb (only sphere):
Weight measured 5 times: 0,030 lb
Weight measured 1 times: 0,032 lb
Weight as a measure obtained using oz (only sphere):
Weight measured 5 times: 0,46 oz
Weight measured 1 times: 0,50 oz
I think the weight error does not vary more than 0.5 grams, at most 1 gram.
The diameter measurement using a template I get it calibrated and counting the millimeters of a millimeter rule that scans while the sphere.
I do not value the irregularities in diameter because they are minimal. Nor do I appreciate about weight because the material is shifted in these irregularities.
Thank you for your interesting questions and contribution to the subject.
Sorry for my English.