Price correlates with labour, yes, but value is not price. The labour theory of value is disputed. It explains why more labour-intensive goods have higher prices, but not why mutually beneficial exchanges could still happen even in the absence of labour and modification of goods, culture has a major impact on demand, demand rises in the expectation of future shortages and demand for a good lowers the more someone has of that good available. The subjective theory of value explains all of these aswell as the correlation of price and labour -the cost of purchasing labour is embedded in the price, and the sale of labour itself involves subjective valuation, the worker thinks he has more to gain with the wage than with the labour time he forfeits and vice versa for his employer. Diamonds have value because their buyers think they have more to gain with the diamond (because of its beauty or whatever, not because they desire it for the labour expended on it) than with the money they have forfeited in its purchase.