1. Lost certificate: Many precious gems were sold at a lower price because their children didn't know the value of the gems because their certificates were lost or their original owners died, which was quite common in the jewelry market in medieval Japan.

2. Market demand: The demand for rubies in Japanese medieval jewelry market is relatively low, which makes the price of rubies in the market relatively low.

3. Quality: The quality and value of ruby are related to its color, cut and purity. Some medieval rubies may have relatively low prices because of quality or cutting problems.

4. Supply exceeds demand: In some periods, the supply of rubies in the market exceeds demand, which may lead to price decline.

5. Competitive pressure: Compared with other jewelry brands or competitors, in order to attract customers, some merchants may sell rubies at lower prices to gain market share.

It should be noted that although the price of Japanese medieval rubies may be relatively low, you should be cautious when buying them. Buying jewelry is a high-risk investment, and it is necessary to carefully check the quality and authenticity of gems. It is best to choose a reputable merchant to buy, and make sure that the formal certificate and guarantee are attached to the purchase.